Red-necked wallabies, Macropus rufogriseus banksianus, spent most of their time alone, and their groups generally were small and unstable. Average group size varied seasonally, but was not influenced by population density. Groups were least stable in winter when rates of movement of feeding wallabies were highest. Individuals were no less alert when in groups than when alone, but rates of movement of grouped individuals were unusually high. Mothers and their independent offspring (subadult sons, and subadult and adult daughters) spent more time together than expected from the degrees to which their home ranges overlapped. Females that associated regularly tended also to breed in synchrony. Males spent more time than expected with other males of their own body sizes. It appears that, despite their relative solitariness and the fluidity of many of their associations, red-necked wallabies have a social organization similar in form to that of many species of more gregarious mammals.