Some words borrowed from other languages, in particular Latin, Greek and French, have irregular plurals in English. Although many now have the usual English ‑s plural, some still retain a Latin, Greek or French plural ending, while others have both. For example, the plural of stimulus is stimuli, the plural of focus is focuses or foci, and the plural of virus is viruses.

Your computer spelling checker will alert you to incorrect plural forms, though in some cases preference for the original or the English ‑s plural differs from subject to subject and between formal and informal language.

Examples of the most common types of irregular plurals in university and research writing are given below.

-a to -ae

Singular Plural
antenna antennae or antennas
formula formulas or formulae
larva larvae
nebula nebulae or nebulas
vertebra vertebrae or vertebras

-eau to -eaux

Singular Plural
bureau bureaus or bureaux
plateau plateaus or plateaux
tableau tableaux

-ies and -ies

Singular Plural
series series
species species

-is to -es

Singular Plural
analysis analyses
axis axes
basis bases
crisis crises
diagnosis diagnoses
hypothesis hypotheses
metamorphosis metamorphoses
nemesis nemeses
neurosis neuroses
oasis oases
parenthesis parentheses
synopsis synopses
thesis theses

-ma to -mata

Singular Plural
angioma angiomas or angiomata
carcinoma carcinomas or carcinomata
h(a)ematoma h(a)ematomas or h(a)ematomata
lymphoma lymphomas or lymphomata
melanoma melanomas also melanomata
sarcoma sarcomas or sarcomata
schema schemata or schemas
stigma stigmas and stigmata
stoma stomata or stomas

-o to -oes

Not all words ending in -o add -e- in the plural, for example embryos, neutrinos, placebos. However, a number of those that do are listed below:

Singular Plural
archipelago archipelagos or archipelagoes
cargo cargos or cargoes
echo echoes
embargo embargoes
fresco frescos or frescoes
ghetto ghettos or ghettoes
halo haloes
hero heroes
mango mangos or mangoes
manifesto manifestos or manifestoes
mosquito mosquitoes or mosquitos
potato potatoes
tomato tomatoes
tornado tornadoes
veto vetoes
volcano volcanoes

-on to -a

Singular Plural
criterion criteria
ganglion ganglia or ganglions
mitochondrion mitochondria
phenomenon phenomena
spermatozoon spermatozoa
taxon taxa

-um to -a

Singular Plural
addendum addenda
aquarium aquariums or aquaria
auditorium auditoriums or auditoria
bacterium bacteria
consortium consortia also consortiums
continuum continua or continuums
curriculum curricula or curriculums
datum data
dictum dicta or dictums
equilibrium equilibriums or equilibria
erratum errata
forum forums or fora
maximum maximums or maxima
medium media
memorandum memorandums or memoranda
millennium millennia or millenniums
minimum minimums or minima
moratorium moratoria or moratoriums
ovum ova
phylum phyla
referendum referenda or referendums
septum septa
serum serums or sera
spectrum spectra or spectrums
stratum strata or stratums
symposium symposia or symposiums
ultimatum ultimatums or ultimata

-us to -i

Singular Plural
bacillus bacilli
bronchus bronchi
focus focuses or foci
fungus fungi or funguses
locus loci
nucleus nuclei or nucleuses
radius radii or radiuses
stimulus stimuli
syllabus syllabuses or syllabi

-us to -ra

Singular Plural
corpus corpora or corpuses
genus genera or genuses

-x to -ces

Singular Plural
apex apexes or apices
appendix appendixes or appendices
codex codices
helix helices or helixes
index indexes or indices
matrix matrices or matrixes
thorax thoraxes or thoraces
vortex vortices or vortexes

Other irregular plurals

Singular Plural
man men
woman women
child children
brother brothers, religious brethren
tooth teeth
foot feet
goose geese
mouse mice
louse lice
ox oxen