This is the British Isles only Pill Bug, and is large, up to 18 mm long and with a much more rounded or domed appearance than the other garden species. The colour varies, but most individuals are slate grey. The shell is much thicker than that of the other woodlice and it is much more waterproof. The animals can therefore inhabit drier places than the other species, and they are often abroad during the daytime. Well-developed pseudotracheae enable them to breathe under such conditions. The thick shell requires plenty of lime however, and therefore pill-bugs are restricted to lime rich areas. Their normal habitats are the dry grasslands of the chalk and limestone in southern England, and they are relatively rare in the north. They are not common garden animals, but they do occur quite frequently around old walls and buildings where crumbling mortar adds lime to the soil.

The pill-bugs ability to roll up into a tight ball, with the tough skeletal plates on the outside, gives it several advantages. Evaporation from the lower surface is greatly reduced when the animals are rolled up, and this helps them survive short periods of drought. The rolled up animals are also much more difficult for the shrews and other small predators to get at, because the balls are too large for them to get their jaws around. The repellent glands of the uropods are less well developed than in other woodlice because there is less need for them.

The pill-bug is quite easy to confuse with the pill-millipede at first, but there are identifiers which make them easy to distinguish apart.

The millipede is generally much blacker and also a more glossy animal and, when it is active, it can be seen to have a lot more than seven pairs of legs, which is the norm in woodlice, 17 - 19 in fact. When the animals are rolled up, the small plates covering the pleon or abdomen of the woodlouse will easily identify it (see picture above), the millipede has no such plates.

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