Grouping behaviour, tail-biting behaviour and sexual dimorphism were investigated in Cordylus cataphractus, a rock-dwelling lizard from South Africa. Eighty-five per cent of individuals collected during different seasons occurred in groups of two or more, which shows that grouping behaviour is a year-round phenomenon in this species. Group size ranged from one to approximately 30. Groups smaller than nine individuals normally included only one adult male, but larger groups often had more than one. Sixty-eight per cent of lone individuals were adult males. All individuals (n= 134) attempted to bite their tails and roll into a ball on collection. Cordylus cataphractus prefers horizontal crevices with an accompanying ledge on which animals can perch in close proximity to the crevice. Males apparently reach larger asymptotic body sizes than females and also have larger heads and longer tails. The higher incidence of scars among males suggests that sexual dimorphism could result from sexual selection, but sexual differences in scaling of head and tail dimensions, however, suggest that the dimorphism may be the result of differential energy allocation by females. Unlike most other Cordylus species which have female-biased sex ratios, the ratio is close to one in C. cataphractus.