The species epithet, which is the second word in a two-part scientific name, is always written in lowercase. The genus name that precedes it is always capitalized. Both parts are italicized. So you write Homo sapiens, not Homo Sapiens and not Homo sapiens. This rule holds across biology, medicine, ecology, and every other field that uses binomial nomenclature, though the details get more interesting once you move beyond animals and plants into bacteria, viruses, and the question of common names.
The Core Formatting Rule
Scientific names for organisms follow a system established by Carl Linnaeus in the eighteenth century: a capitalized genus name followed by a lowercase species epithet, with the whole name set in italics. Escherichia coli, Tyrannosaurus rex, Cannabis sativa, Quercus alba. No exceptions exist to the lowercase rule for the species epithet, even when it derives from a proper noun. A species named after Charles Darwin becomes darwinii, not Darwinii, and a species named after Japan becomes japonica, not Japonica. The formal codes of nomenclature treat scientific names as Latin words regardless of their actual linguistic origin, and Latin species epithets are always lowercase.
This convention applies identically whether the name appears in a title, a heading, or the middle of a sentence. Some writers instinctively capitalize every word in a paper title, which creates the temptation to write “The Ecology of Quercus Alba in Eastern Forests.” That would be wrong. The species epithet stays lowercase even in title case.
What About the Genus Alone?
When you refer to the genus without specifying a species, the genus name stands alone, capitalized and italicized: Staphylococcus, Drosophila, Salmonella. After the first full mention of the binomial, most style guides allow you to abbreviate the genus to its initial letter followed by a period. So Escherichia coli becomes E. coli on second reference, and Staphylococcus aureus becomes S. aureus. The abbreviation keeps the italics and keeps the period. One common mistake is dropping the period or failing to italicize the abbreviated form.
A subtlety worth knowing: if your paper discusses two genera that start with the same letter, abbreviating both to a single letter creates ambiguity. A paper about Staphylococcus aureus and Streptococcus pyogenes should not abbreviate both to S. because the reader cannot tell which genus is meant. In that situation, most journals require you to use at least enough letters to distinguish them, such as Staph. and Strep., or to spell out both names every time.
Ranks Above and Below the Species Level
Taxonomic ranks higher than genus follow a different convention. Kingdom, phylum, class, order, and family names are capitalized but not italicized. So you write Animalia, Chordata, Mammalia, Primates, and Hominidae in roman (upright) type with an initial capital letter. This pattern is consistent across all the major nomenclature codes.
Below the species level, subspecies and variety names follow the same formatting as the species epithet: lowercase and italicized, appended after the species epithet. The Bengal tiger is Panthera tigris tigris, with the subspecific epithet in lowercase italics just like the species epithet. Botanical writing sometimes inserts an abbreviation for the rank, as in Brassica oleracea var. capitata (cabbage), where “var.” is in roman type but the variety epithet is italicized and lowercase.
One area that trips people up is the transition between formal Latin ranks and their anglicized versions. “Hominidae” is a formal taxonomic family name: capitalized, roman type. But “hominid,” the informal English derivative, is lowercase and roman. Likewise “Primates” is the order name, but “primates” is just an ordinary English word. Whenever you turn a Latin rank into an adjective or a casual noun, it loses its capital letter.
How Bacterial Names Follow the Same Logic
Bacterial nomenclature operates under the International Code of Nomenclature of Prokaryotes, which mirrors the conventions used for animals and plants. Genus names are singular nouns that start with a capital letter. Species epithets are lowercase and must agree in grammatical gender with the genus name, since the names are treated as Latin even when they derive from other languages.1ScienceDirect (Elsevier). Bacterial nomenclature in the era of genomics So Mycobacterium tuberculosis follows the standard pattern: capitalized genus, lowercase epithet, both in italics.
Bacterial names come up constantly in medical and clinical writing, and a common error in manuscripts is to drop the italics or capitalize the species epithet, especially when a name has become very familiar. You see “Staphylococcus Aureus” surprisingly often in draft manuscripts, and it is always wrong. The epithet aureus (golden, referring to the color of the bacterial colonies) stays lowercase regardless of how important the pathogen is.
Viruses Play by Different Rules
Virus nomenclature is where things genuinely diverge from the standard pattern, and even experienced scientists get confused here. The International Committee on Taxonomy of Viruses (ICTV) distinguishes between a virus species name and a virus name, and the formatting rules are different for each.
A virus species name, which is the formal taxonomic designation, is italicized with the first letter capitalized. So the species name is written as Rabbit hemorrhagic disease virus. But the virus name itself, used in everyday scientific prose, is written in regular (non-italicized) type and entirely in lowercase unless part of the name is a proper noun. That gives you “rabbit hemorrhagic disease virus” in running text and Rabbit hemorrhagic disease virus when you are specifically referring to the taxonomic species.2PubMed Central. Clarification and guidance on the proper usage of virus and virus species names
This distinction matters because most papers use the virus name, not the species name, when discussing a pathogen in context. You would write “human immunodeficiency virus” in lowercase roman type throughout your paper, even though the formal species name Human immunodeficiency virus 1 would be italicized and capitalized. The proper noun exception means “Ebola virus” keeps its capital E (because Ebola is a place name), even though the rest of the virus name is lowercase. Similarly, “West Nile virus” capitalizes “West Nile” because it is a geographic proper noun, but “virus” stays lowercase.
The confusion is understandable, because no other branch of biology makes the same distinction between a species name and a colloquial name with different formatting rules that both appear in formal scientific papers. If you are writing about viruses and are unsure, the safest approach is to check the ICTV’s current taxonomy for the formal species name and use the non-italicized, lowercase virus name for running text.
Common Names and Why the Rules Vary by Field
Scientific names have clear, universal formatting rules. Common names do not. Whether to capitalize the common English name of an organism depends entirely on what field you are working in and sometimes on the specific journal or style guide.
Ornithologists have long capitalized the English common names of bird species: “American Robin,” not “American robin.” The logic is that capitalization makes the standardized common name visually distinct from a casual description. “American Robin” refers to the specific species Turdus migratorius, while “American robin” could be read as any robin that happens to be American. This convention is now deeply entrenched in ornithological journals, field guides, and birding organizations.
Entomologists follow a similar convention for insects with approved common names. The Entomological Society of America maintains an official list, and names on that list are capitalized: “European Corn Borer,” “Asian Tiger Mosquito.” Mammals, fish, and herpetological species have more fragmented practices. Some organizations standardize capitalized common names; many do not.
In Canada, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) adopted a blanket rule that applies across all taxonomic groups: capitalize the first letter of each word in the English common name.3COSEWIC. Guidelines for Naming Wildlife Species Under this approach, you write “White-Tailed Deer” and “Northern Leopard Frog” regardless of the taxonomic group. The same guidelines note that French common names follow a different convention, using lowercase except for birds, where only the first word and proper nouns are capitalized.3COSEWIC. Guidelines for Naming Wildlife Species This is a good illustration of how arbitrary these conventions can be: even within a single document, the rules shift depending on the language.
If you are writing for a general audience rather than a specialized journal, most mainstream style guides (AP, Chicago, etc.) do not capitalize common names of animals or plants. A newspaper article about white-tailed deer keeps it lowercase. A COSEWIC status report capitalizes it. Neither is wrong; they follow different conventions for different purposes.
Names Derived from People and Places
A persistent misconception is that species epithets derived from proper nouns should keep their capital letter. They should not. Under the zoological code, a species epithet is always lowercase regardless of its etymology. Under the botanical code, historical practice allowed capitalizing epithets derived from personal names or former genus names, but modern botanical writing has moved toward universal lowercase, and most journals now enforce it.
Scientific names are formally treated as Latin words, and their formation often involves Latinizing words from any language, adding suffixes for grammatical agreement.4Proceedings of the Royal Society B: Biological Sciences. Naming the menagerie: creativity, culture and consequences in the formation of scientific names The person’s name or place name that inspired the epithet gets absorbed into the Latin machinery. Darwin becomes darwinii. Victoria becomes victoriae. The Galápagos Islands become galapagoensis. Once Latinized, the epithet follows Latin conventions, which means lowercase.
The same rule applies to bacterial epithets. A bacterium named after Louis Pasteur is Pasteurella at the genus level (capitalized, because genus names are always capitalized), but a species epithet honoring him would be lowercase: pasteurii.1ScienceDirect (Elsevier). Bacterial nomenclature in the era of genomics The distinction between genus-level capitalization and epithet-level lowercase catches people off guard, but the principle is consistent: the position in the binomial determines the capitalization, not the prestige of the person being honored.
When Names Become Everyday Words
Some scientific names have crossed into ordinary English and lost their formatting along the way. Nobody italicizes “salmonella” in a newspaper headline about a food recall, and writing “E. coli” in roman type in a consumer-facing health advisory is standard practice. The genus Staphylococcus becomes “staph” in a hospital hallway. These vernacularized forms follow English grammar rather than Latin: they are lowercase, roman type, and often abbreviated or modified in ways that would be impermissible in a taxonomic context.
The question of when a scientific name has become sufficiently vernacularized to shed its italics and capitalization has no universal answer. In a research paper, you should always use the formal formatting: Salmonella enterica, Escherichia coli. In a patient information leaflet, “salmonella” and “E. coli” in plain type are perfectly acceptable. The context determines the register, and the register determines the formatting.
A related case is genus names used as informal group descriptors. “The staphylococci” (plural, lowercase, roman) refers loosely to bacteria in the genus Staphylococcus. “Streptococci,” “clostridia,” and “pseudomonads” all work this way. These are English words derived from Latin taxonomic names, and they follow English conventions once they have been informally adopted. In a formal taxonomic context, you would write Staphylococcus spp. (with “spp.” indicating multiple species in the genus, in roman type). In clinical shorthand, “staph” does the job.
Mistakes Reviewers Catch Most Often
Errors in species name formatting are among the most common typographical issues in biological and medical manuscripts.5Europe PMC. Common mistakes when using plant names and how to avoid them. The mistakes cluster into a few predictable categories:
- Capitalizing the species epithet: Writing Rosa Canina instead of Rosa canina. This is by far the most frequent error, especially with plant names that have become culturally prominent.
- Dropping italics: Writing Homo sapiens in roman type. Some word processors lose italics during copy-paste or format conversion, so this often reflects a production error rather than ignorance of the rule.
- Inconsistent abbreviation: Abbreviating the genus on first mention, or abbreviating it to a single letter when two genera in the same paper share that initial.
- Mixing virus and species conventions: Italicizing a virus name in running text when only the formal species name should be italicized, or capitalizing the virus name when it contains no proper noun.
- Capitalizing anglicized derivatives: Writing “Staphylococcal infection” instead of “staphylococcal infection.” Once a taxonomic name has been turned into an English adjective, it loses its capital.
Automated spell-checkers are no help here and sometimes make things worse, because most word processors flag lowercase Latin epithets as misspelled and auto-correct them to capitalized forms. If you are writing a paper with many species names, adding them to your custom dictionary or turning off auto-capitalization for those terms saves time during revision.
How Formatting Rules Differ Across the Nomenclature Codes
Three major international codes govern the naming of organisms, and while they agree on the basics of capitalization, they differ in some details worth knowing if you work across disciplines.
The International Code of Zoological Nomenclature governs animal names. It mandates the binomial system with a capitalized genus and lowercase epithet, and it does not permit capitalization of the epithet under any circumstances. The International Code of Nomenclature for algae, fungi, and plants (commonly called the botanical code) historically allowed an optional capital for epithets derived from personal names or former generic names, but this provision has fallen out of favor, and most botanical journals now require universal lowercase. The International Code of Nomenclature of Prokaryotes handles bacteria and archaea, and its rules on capitalization and gender agreement are essentially identical to the zoological and botanical codes.1ScienceDirect (Elsevier). Bacterial nomenclature in the era of genomics
The ICTV’s virus taxonomy operates somewhat independently, with the species-name-versus-virus-name distinction described earlier.2PubMed Central. Clarification and guidance on the proper usage of virus and virus species names Because viruses do not fit neatly into the Linnaean system, their nomenclature has evolved its own conventions. If you are writing a paper that spans multiple domains of life, be aware that the formatting rules for your viral subjects may differ from those for your bacterial or animal subjects, even within the same manuscript.
Formatting in Reference Lists and Databases
Reference managers and databases introduce their own formatting headaches. PubMed, for instance, strips italics from titles in its default display, which means species names in paper titles often appear in roman type when you search the database. This is a display limitation, not a reflection of the actual formatting in the published paper. If you are copying a title from PubMed into your reference list, you need to manually restore the italics for any species names.
GenBank, the NCBI’s sequence database, stores organism names in plain text without italics. BOLD Systems and other biodiversity databases display names in italics on their web interfaces but may export them in plain text. The point is that databases are storage systems, not style authorities. The formatting rules live in the nomenclature codes and journal style guides, not in whatever a database happens to display.
Automated tools for species-name recognition in text mining rely on the capitalization convention to flag potential taxonomic names. A capitalized word followed by a lowercase word, both in italics, is a strong signal that a binomial species name is present. Inconsistent formatting in papers does not just annoy reviewers; it can cause a species to be missed entirely by text-mining algorithms that scan the literature for biodiversity data. Getting the formatting right has implications beyond aesthetics.