The head and body of the brown rat measure 23 centimetres, and its tail up to 23 centimetres. It is brownish grey in colour above, and pale brown to black and pale grey below. It has a scaly and bare tail, pink pads and short ears.
The brown rat is found around human settlements, especially farms, sewers and rubbish dumps, basements and storehouses. Distributed worldwide, it has been accidentally introduced by humans to many wild habitats, causing irreversible damage to local fauna on isolated islands.
Generally nocturnal, the brown rat rests in shallow tunnels during the day. Although omnivorous, it prefers grains and cereals. It occasionally attacks other small animals. It climbs, swims and dives well. Usually living in family groups of 60 individuals, it builds nests in human houses or digs systems of tunnels in the ground. One and the same nest might be used by a number of females taking care of their young together. It breeds about five times throughout the year, producing six to 12 babies per brood.