Completely harmless. It has no bite, no sting and no venom, it does not damage a house, and it cannot survive indoors for long. It is a crustacean living on land.
Size10–18 mm
MarkingsRolls into a tight, complete sphere when disturbed — the single decisive character · Slate-grey, strongly domed, with overlapping armoured plates
WhereUnder stones, logs, pots and leaf litter — anywhere dark and reliably damp. Turns up indoors in basements and around door thresholds.
SeasonActive whenever it is mild and damp; least visible in dry heat
How to tell it apart
Rolls into a tight, complete sphere when disturbed — the single decisive character
Slate-grey, strongly domed, with overlapping armoured plates
Seven pairs of legs, hidden entirely when rolled
Two short antennae, no tail projections at the rear
Up to about 18 mm, and noticeably rounder than a sow bug
Where you’ll find it
Across the whole United States. It is a European species that arrived with people and is now one of the most widespread land invertebrates in the country.
Under stones, logs, pots and leaf litter — anywhere dark and reliably damp. Turns up indoors in basements and around door thresholds.
Active whenever it is mild and damp; least visible in dry heat.
How it behaves
It breathes through gill-like structures that must stay moist, which explains almost everything it does: it hides by day, comes out at night, and dies within a day or two in a dry room. Rolling into a ball is a defence against both predators and drying out. It eats decaying plant matter and is a genuine contributor to composting, and it moults in two halves — back first, then front — so a half-pale, half-dark individual is mid-moult rather than ill.
You’ve found one indoors
Nothing, other than putting it back outside. Repeated appearances indoors are a moisture signal rather than an infestation — check for damp around thresholds, basement walls and under sinks. They cannot breed indoors in any meaningful way, so the ones you find have walked in and are dying.
Common questions
Is a pill bug an insect?
No. It is a crustacean, more closely related to a crab or a shrimp than to any insect, and it breathes through gills. Seven pairs of legs also rules out an insect, which has three.
What is the difference between a pill bug and a sow bug?
Rolling. A pill bug curls into a complete sphere; a sow bug flattens itself and runs, and has two small tail projections at the rear. Both are harmless.
Do pill bugs bite?
No. They have no biting mouthparts capable of it and no interest in people. They are safe for children to handle.
Why are they in my house?
They wandered in looking for damp and will not last long. Finding them repeatedly points at moisture getting in somewhere, which is the thing worth fixing.
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