Plural of “louse”

The plural of louse is lice.

Why

"Louse" descends from Old English "lus," and its plural "lice" comes from an ancient Germanic vowel-mutation pattern — the same process that gives us "mouse/mice," "goose/geese," and "foot/feet." Rather than adding an ending, these words shift their internal vowel to signal plurality, a leftover from a system of plural formation that predates the -s ending's dominance in English by many centuries.

This mutation pattern, called umlaut plurals by linguists, was genuinely productive across a much wider set of Old English nouns centuries ago; most of those words have since regularized to take -s, leaving "lice," "mice," "geese," "feet," "teeth," and a small handful of others as surviving fossils of a rule that used to apply far more broadly.

Usage note

"Lice" is used identically whether describing head lice, body lice, or lice infesting other animals or plants (as in "plant lice," a common name for aphids) — the mutation plural covers all these senses without any variation.

The word carries a strong negative or clinical connotation in most contexts (school lice checks, medical treatment for infestations), and this is one of the few mutation-plural nouns whose singular form, "louse," has also developed a figurative insult sense ("he's a real louse"), whose own plural, interestingly, regularizes to "louses" rather than "lice" when used this figurative way. Old English preserved a whole small set of these mutation plurals through a sound-shift process linguists call i-mutation, where a following vowel long since lost from the word's ending nonetheless changed the vowel before it, permanently altering "lus" toward "lys" (an early form of "lice") — the vowel change is the fossilized trace of a sound that vanished from pronunciation many centuries ago.

Common real mistakes

Writers occasionally, and incorrectly, pluralize the figurative insult sense as "lice" ("those lice cheated us") — but when "louse" means a contemptible person rather than the insect, the regular plural "louses" is the standard form, a genuine and useful exception to know.

The wider pattern

"Louse/lice" belongs to the small closed class of English mutation plurals — "mouse/mice," "goose/geese," "foot/feet," "tooth/teeth," "man/men," "woman/women" — nouns that never adopted the -s ending and instead preserve a much older Germanic pattern of internal vowel change. Head lice specifically have become resistant to some common over-the-counter treatments in many regions, a public health concern that has driven renewed research into lice biology and alternative treatments over the past two decades, entirely separate from the word's grammatical curiosities but a reminder of how ordinary vocabulary can intersect with genuinely active scientific research.

Examples

• The school nurse checked every student's hair for lice that week.
• Head lice spread quickly among the kindergarten class.
• The farmer treated the flock for lice before the cold season began.
• Researchers study how lice have evolved resistance to common treatments.
• Several species of lice infest different parts of the human body.
• The dog was scratching constantly because of a bad case of lice.
• Plant lice, better known as aphids, damaged much of the garden's roses.
• The book describes delousing procedures used in wartime field hospitals.
• Lice cannot survive long away from a host's body heat.
• Her younger brothers were both notorious louses growing up, always breaking promises.
• The camp had to be treated for lice after the outbreak spread.
• Combing regularly can help catch lice before an infestation grows.
• Historical accounts describe soldiers plagued by lice in the trenches.
• The vet checked the barn cats for both fleas and lice.
• Public health officials tracked treatment-resistant lice across several school districts.
• The veterinarian recommended a specific shampoo for lice on farm animals.
• Historical letters mention entire households battling lice during long voyages.
• New research is examining why some lice populations resist standard treatments.