How to Write Binomial Nomenclature Correctly

Binomial nomenclature follows a straightforward formula: a capitalized genus name followed by a lowercase species name, with both words italicized. Write Homo sapiens, not Homo Sapiens or homo sapiens. That core rule covers most situations, but scientific names come with a surprising number of formatting edge cases that trip up students, researchers, and writers alike. The conventions shift depending on whether you are writing by hand or typing, whether you have already mentioned the species in your text, and even whether you are naming an animal, a plant, or a virus.

The Two Foundational Rules

Every correctly written binomial name satisfies two non-negotiable conditions. First, the genus (the first word) is always capitalized, and the specific epithet (the second word) is always lowercase. Second, the entire two-word name is italicized. The genus is capitalized because it functions like a proper noun, identifying a group of closely related organisms. The specific epithet is never capitalized, even when it derives from a person’s name or a place. The eastern red cedar, for instance, is Juniperus virginiana, not Juniperus Virginiana, despite the obvious reference to Virginia.¹ Domestic cattle are Bos taurus, never Bos Taurus.¹

This lowercase rule for the specific epithet is universal in zoology and has been standard in botany since the 1990s, though you will occasionally encounter older botanical texts that capitalize epithets derived from proper nouns. Modern style guides and the current International Code of Nomenclature for algae, fungi, and plants all agree: keep it lowercase.

Italics, Underlining, and When Each Applies

If you are typing on a computer, italicize. There is no acceptable alternative in digital or printed text. When you are handwriting, though, producing true italics is impractical, so the convention is to underline both words instead. The underline serves as a visual stand-in for italics, signaling to anyone reading your notes or a handwritten manuscript that these words are a formal Latin name.¹ In typed work, underlining looks odd and outdated. Use it only on paper.

One formatting detail that catches people off guard: the italicization applies only to the Latin binomial itself, not to any surrounding taxonomic information. If you write the authority (the person who named the species) after the binomial, the authority name stays in regular (roman) type. So you would write Escherichia coli (Migula 1895) Castellani and Chalmers 1919, with only the first two words italicized. The same goes for higher taxonomic ranks like family or order names, which are capitalized but not italicized in zoological writing, though botanists do italicize all ranks from genus downward in some contexts.

Abbreviating After First Mention

In any piece of writing, the first time you mention a species you spell out the full binomial: Escherichia coli. Every subsequent mention in the same document can abbreviate the genus to its initial letter: E. coli. The period after the initial is mandatory. This convention keeps text readable without sacrificing clarity, as long as no two genera in your text share the same initial. If you are writing about both Staphylococcus aureus and Streptococcus pyogenes, abbreviating both to S. creates ambiguity. In that case, use a longer abbreviation for one of them, such as Staph. aureus and Strep. pyogenes, or spell the genus out each time.

Note that the abbreviated form still gets italicized: E. coli, not E. coli. And the specific epithet should never appear alone without some form of the genus in front of it. Writing just “coli” in reference to E. coli is technically meaningless because the epithet only identifies a species within its genus.

What “sp.” and “spp.” Mean and How to Format Them

Sometimes you know the genus but not the exact species. In that case, you write the genus name followed by “sp.” for a single unidentified species or “spp.” for multiple unidentified species: Plasmodium sp. or Plasmodium spp. The key formatting point here is that “sp.” and “spp.” are not italicized. They are abbreviations of the Latin word “species,” not part of the formal name, so they stay in roman type. The genus remains italicized; the abbreviation does not.

A similar convention applies to “cf.” and “aff.”, which indicate uncertainty. If you write Canis cf. lupus, you are saying the organism resembles a wolf but has not been conclusively identified as one. Again, the Latin abbreviation sits in roman type between the italicized genus and the italicized epithet. These markers are common in field biology, paleontology, and ecological surveys where definitive identification is not always possible.

Subspecies and Ranks Below Species

Binomial nomenclature is technically two names, but biology recognizes finer divisions. A subspecies adds a third term: Homo sapiens sapiens for modern humans versus the extinct Homo sapiens idaltu. All three words are italicized. In zoology, the subspecific epithet simply follows the species epithet with no connecting word. In botany, you insert a rank indicator that is not italicized: Lavandula angustifolia subsp. angustifolia. The “subsp.” (or “var.” for variety, “f.” for form) sits in roman type, just like “sp.” does.

This distinction between zoological and botanical practice matters if you are writing across disciplines. A zoologist would write Ursus arctos horribilis with no rank marker, while a botanist describing a plant variety includes one. Neither approach is wrong; they follow different codes of nomenclature.

Authority Citations and When You Need Them

In formal taxonomic writing, the binomial is often followed by the name of the person who first described the species, sometimes with a year. Tyrannosaurus rex Osborn, 1905 tells readers that Henry Fairfield Osborn described the species in 1905. In botany, authority abbreviations follow standardized forms: Rosa canina L. means Linnaeus described the dog rose. If the species has been moved to a different genus since it was first named, the original describer’s name goes in parentheses: Prunus dulcis (Mill.) D.A. Webb indicates that Miller first named the almond, and Webb later transferred it.

Most non-taxonomic writing omits the authority entirely. If you are writing a research paper in ecology, a field guide, a lab report, or a news article, the binomial alone is typically sufficient. Authorities become important in taxonomic revisions, checklists, and databases where you need to distinguish between species that may share a name or have been reclassified. The Global Biodiversity Information Facility (GBIF), for instance, stores the full scientific name including authorship in its data records to avoid confusion with autonyms, hybrid formulas, and other non-trivial binomials.²

How Viruses Break the Rules

For most of the history of virology, viruses did not follow binomial nomenclature at all. Species names were freeform English phrases like “Ebola virus” or “Human immunodeficiency virus 1,” written in regular type with no italics. That changed recently. Starting in 2021, the International Committee on Taxonomy of Viruses (ICTV) mandated that virus species names be written as true binomials, with a genus name ending in “virus” followed by a one-word species epithet. The Ebola virus species, for example, became Orthoebolavirus zairense. Some of the new epithets are Latinized, while others are freeform coinages, as with Simplexvirus humanalpha1 for the herpes simplex virus.³

Even with the binomial format, virus nomenclature differs from zoological and botanical naming in a fundamental way. The ICTV draws a strict line between the species as a taxonomic category and the virus as a physical entity. The species name Orthoebolavirus zairense is italicized because it refers to the formal taxonomic concept. But the informal virus name “Ebola virus” is still used in non-taxonomic contexts (clinical papers, news reports) and is written in regular roman type with no italics.⁴ This dual-naming system confuses plenty of people, so if you are writing about viruses, pay attention to whether you mean the formal species or the colloquial virus name, and format accordingly.

The “Candidatus” Problem in Microbiology

Microbiologists face another formatting wrinkle. The traditional rules of prokaryotic nomenclature require that a bacterium or archaeon be cultured in a lab before it receives a valid scientific name. But modern genomics constantly discovers organisms known only from environmental DNA sequences, things that have never been grown in pure culture. To handle this, the prefix “Candidatus” was introduced in the mid-1990s. You write Candidatus Pelagibacter ubique to indicate that the organism has not been formally validated under the International Code of Nomenclature of Prokaryotes.⁵

The formatting convention is that “Candidatus” is italicized and the rest of the name follows standard binomial rules, but the entire name is often placed in quotation marks to signal its provisional status. However, the handling has been inconsistent across journals and databases, partly because “Candidatus” names do not have formal standing in the nomenclatural code.⁵ If you are writing about uncultured microbes, check the target journal’s style guide. Some require the quotation marks, others do not, and the field is still sorting out a permanent solution.

Mistakes That Come Up Constantly

Certain errors appear so often that they are worth listing explicitly:

Capitalizing the species epithet: Writing Tyrannosaurus Rex instead of Tyrannosaurus rex. This is probably the single most common mistake, and autocorrect on phones and computers makes it worse by “fixing” the lowercase word.
Forgetting italics entirely: Homo sapiens in plain text is technically incorrect in any typed document. If your software supports formatting, use it.
Italicizing non-name elements: Writing sp. or var. in italics when these should be roman. Same goes for authority names after the binomial.
Dropping the genus on second reference: Writing just “sapiens” or “rex” without the abbreviated genus. The epithet alone is not a valid reference.
Treating common names as scientific names: “E. coli” is an abbreviated scientific name and gets italicized. “Staph infection” is a colloquial shortening and does not.
Using outdated synonyms: Species get reclassified. If you are writing about a species that has been moved to a new genus, use the current accepted name. Free tools exist to check this.

Autocorrect deserves special mention because it actively works against you. Most word processors will capitalize the first letter of any word following a period, which means E. coli often becomes E. Coli. Disable auto-capitalization for scientific documents, or get into the habit of checking every binomial before you submit.

Tools That Check Your Names for You

If you are working with long species lists, manually checking every name is tedious and error-prone. Several web tools can automate the process. Plantminer, for instance, searches for current accepted names, synonyms, correct spellings, and higher classification for seed plants, reducing the grunt work of compiling and formatting botanical checklists.⁶ For animals, the Integrated Taxonomic Information System (ITIS) and the Catalogue of Life serve similar functions. For bacteria, the List of Prokaryotic names with Standing in Nomenclature (LPSN) is the standard reference. These databases not only catch typos but also flag outdated names and provide current synonyms, so you can be confident your nomenclature reflects the latest taxonomic consensus.

GBIF’s publishing toolkit also standardizes how names are shared across databases, recommending that records always include the full scientific name with authorship even when the name is also broken into component fields.² If you are contributing data to a biodiversity platform, including the authority string helps downstream users distinguish between homonyms and correctly link your data to the right taxon.

When Common Names Are Acceptable

Not every mention of a species requires the Latin binomial. In general-audience writing, a common name is often clearer: “grizzly bear” communicates better to most readers than Ursus arctos horribilis. The convention in scientific writing is to introduce the organism by its common name followed by the binomial in parentheses on first mention, then use whichever form is more natural for your audience afterward. In formal papers, you typically switch to the abbreviated binomial. In popular science, you typically stick with the common name.

The problem arises when common names are ambiguous. “Buzzard” refers to hawks in North America and to vultures in parts of Europe. “Cedar” covers trees in completely different families depending on the continent. In any context where precision matters, the binomial is not optional. It is the only name that points to exactly one species worldwide, regardless of the reader’s language or region.

The Ethics and Politics of Species Names

Binomial nomenclature is sometimes treated as a neutral, purely scientific system, but the names themselves carry cultural baggage. A recent analysis found that roughly 24% of African vertebrate birds, reptiles, amphibians, and mammals listed on the IUCN Red List of Threatened Species are named after white, male Europeans from the 19th and 20th centuries.⁷ These honorific names were often assigned during the colonial era, sometimes by collectors who had exploitative relationships with the regions and peoples where the organisms were found. There is a growing movement within taxonomy to rename species whose names are considered offensive or ethically problematic, though changing established names runs into procedural resistance because nomenclatural codes generally prioritize stability.

This debate does not change how you format a binomial, but it does mean the name you find in an older reference might not be the one currently preferred by the scientific community. If you are writing about a species whose name is under active discussion, checking a current database rather than relying on a decades-old field guide is especially important. The formatting stays the same regardless of whether a species eventually gets renamed: genus capitalized, epithet lowercase, both italicized.

Quick-Reference Formatting for Different Ranks

Because different taxonomic levels follow slightly different conventions, here is a practical breakdown:

Kingdom through family: Capitalized, not italicized in zoology (Animalia, Chordata, Felidae). Some botanical journals italicize all ranks from genus down.
Genus alone: Capitalized and italicized (Canis). When referring to the genus as a group rather than a specific species, no epithet is needed.
Species: Genus capitalized, epithet lowercase, both italicized (Canis lupus).
Subspecies (zoology): Third epithet added, all three words italicized (Canis lupus familiaris).
Subspecies/variety (botany): Rank indicator in roman type between italicized name parts (Brassica oleracea var. capitata).
Undetermined species: Genus italicized, “sp.” or “spp.” in roman type (Canis sp.).

If you internalize these patterns, you can format any organism’s name correctly without needing to look it up each time. The system was designed to be consistent, and once the logic clicks, the only real enemies are autocorrect and force of habit.

1
Vanderbilt University Libraries. BSCI 1510L Literature and Stats Guide: 3.1 Specific details regarding scientific writing
2
GBIF Integrated Publishing Toolkit. Best Practices in Publishing Species Checklists
3
PubMed Central. Binomial names for virus species: the rediscovery of an old idea
4
Virus Evolution. A critique of the use of species and below-species taxonomic terms for viruses—time for change?
5
PubMed Central. Nomenclature of prokaryotic ‘Candidatus’ taxa: establishing order in the current chaos
6
Environmental Modelling & Software. Plantminer: A web tool for checking and gathering plant species taxonomic information
7
Proceedings of the National Academy of Sciences (PNAS). Stop using racist, unethical, and inappropriate names in taxonomy