Common Woodlouse

Oniscus asellus

Common Woodlouse

Oniscus asellus

Harmless

Not dangerous

Harmless. No bite, no sting, no venom, and no damage to a building. It eats decaying plant matter and needs damp to breathe, which is why finding one indoors says more about the room than about the animal.

Size12–16 mm

MarkingsFlattens and runs when disturbed rather than rolling into a ball · Shiny grey-brown with pale mottled patches along the edges

WhereLeaf litter, under logs and stones, compost heaps, and the damp margin where a wall meets soil.

How to tell it apart

Flattens and runs when disturbed rather than rolling into a ball
Shiny grey-brown with pale mottled patches along the edges
Two small pointed projections at the rear end, absent on a pill bug
Seven pairs of legs and two obvious antennae
Up to about 16 mm, flatter and more oval than a pill bug

Where you’ll find it

Across the United States, and one of the commonest land isopods in the northeast and Pacific northwest in particular.

Leaf litter, under logs and stones, compost heaps, and the damp margin where a wall meets soil.

How it behaves

A land crustacean that never fully escaped water: it breathes through modified gills that have to stay moist, so it shelters by day and forages at night on decaying leaves and wood. It is an important early stage in breaking down leaf litter, and in a compost heap it is doing useful work rather than causing harm. Females carry eggs in a fluid-filled pouch beneath the body, which is why one occasionally looks oddly swollen underneath.

You’ve found one indoors

Put it outside and think about damp. Woodlice indoors are a symptom, not a cause — they cannot breed in a dry house and are usually coming in under a door or through a gap at ground level near something wet. Nothing needs spraying.

Common questions

Do woodlice damage a house?

No. They eat decaying plant material, not sound timber, and they cannot chew structural wood. If you have them and damp together, the damp is the problem and they are the signal.

Why does this one not roll into a ball?

Because it is a sow bug rather than a pill bug. Only pill bugs curl into a sphere. This one flattens and runs, and has two small tail projections a pill bug lacks.

Are woodlice good for the garden?

Yes, on balance. They accelerate the breakdown of leaf litter and compost. Occasional nibbling on soft seedlings is the only real cost.

Commonly confused with

Pill Bug Harmless
Powder Orange Isopod Harmless

Side-by-side comparisons

Woodlouse vs Pill Bug

Keep identifying

Identify another animal from a photo or return to the Isopods guide.

Previous: Pill Bug · Next: Rubber Ducky Isopod

Where this was checked

iNaturalist — Oniscus asellus checked 2026-07-30

Last reviewed 2026-07-30. We identify, we don’t diagnose — what we can and can’t tell you.