Despite its name, it exhibits several colour forms. It is usually black to light brown in colour with a lighter underside. A typical rat will be 15–20 cm (5.9–7.9 in) long with a further 20 cm (7.9 in) of tail. It is nocturnal and omnivorous, with a preference for grains and fruit. Compared to the Brown Rat, it is a poor swimmer, but more agile and a better climber, tending even to flee upwards. In a suitable environment it will breed throughout the year, with a female producing three to six litters of up to ten young. Females may regulate their production of offspring during times when food is scarce, throwing as few as only one litter a year. R. rattus lives for about 2–3 years. Social groups of up to sixty can be formed.
Black Rats (or their ectoparasites) are able to carry a number of pathogens, of which bubonic plague (via the rat flea), typhus, Weil's disease, toxoplasmosis and trichinosis are the most well known.
In the 1920s in England, several colour variations were bred and shown alongside domesticated brown rats. This included an unusual green tinted variety. Today however, very few people keep Black Rats as pets. Most pet rats (or fancy rats) are domesticated brown rats.