SUMMARYWild rats(Battus norvegicusBerkenhout andR. rattusL.) were kept in large cages fitted with nest boxes and supplied with excess food and water. Colonies varied in size from four to twenty rats, and were usually maintained for at least nine weeks. The rats were individually identified and their behaviour studied in detail.Rattus norvegicus.In all‐male colonies there was a low mortality and most of the rats increased in weight. In colonies containing both males and females, mortality was high among the males, very low among the females; most of the males declined in weight, though some grew well. There was however no fighting for females. It is suggested that the extra fighting which took place in these colonies resembled displacement behaviour and was due to excitement evoked by the presence of females combined with frequent frustration.There were no deaths in all‐female colonies, or in colonies of litter mates. Unless parturient, normal females did not display aggressive behaviour except in ‘play’.Males, but not females, added to established colonies, were attacked by resident males and usually died, sometimes within a few hours and often without visible injury.At the beginning of an experiment there was much exploration, both of the cage and of the other rats. Social relationships were established early. Male members of the colonies fell into three classes: (i) alphas, which were the equals or superiors of other males in the colony, and attacked newcomers; (ii) betas, which adapted themselves to a subordinate role, and were ingratiating towards newcomers; (iii) omegas, which were persecuted by one or more alphas and soon died. Alphas and betas gained in weight; omegas lost weight. There were more omegas in male‐female colonies than in those consisting only of males. Amicable behaviour (apart from coitus and care of young) was shown by both sexes and was based mainly on contact ‘releasing’ stimuli: the most specific of a number of amicable responses was crawling under the belly of another rat. Fighting involved a series of stereotyped acts, including tooth chattering and a characteristic threat posture; there was much wild leaping, but biting produced only superficial damage.Rattus rattus.A small number of experiments withR. rattusshowed that this species possesses all the components of amicable and aggressive behaviour observed inR. norvegicus, but that it is less fierce and more agile. There was no evidence of extra conflict in male‐female colonies. All‐male colonies containing both species were usually peaceful.R. rattusof either sex added tonorvegicuscolonies were usually attacked, but not with such intensity or consistency as were malenorvegicus; if attacked, they died.R. norvegicusmales added torattuscolonies were sometimes attacked, but they did not die.Displacement behaviour and abnormaľ sexual and aggressive behaviour were observed in both species.General.There is no reason to think that dominance hierarchies ever develop in wild rat colonies. Members of a single family do not attack each other. The fighting of wild rats is essentially territorial, not for any specific object. Aggression is most readily evoked in males established in a familiar area, faced with a strange adult male of the same species. In normal females aggression occurs only in defence of a nest containing young. Social responses consist of stereotyped behaviour patterns which are probably innate; but learning plays an important part in determining the choice of response and in secondary adjustments of behaviour to partic