All-day observations of focal individuals were analysed to compare grouping and ranging patterns and the proportion of time spent feeding by females and males; sexually receptive females were not included. Females spent most of their time alone, whereas males spent most of their time in parties with other males. Females travelled shorter distances than males, and spent their time in smaller core areas: when they joined parties, however, they often travelled outside their normal core areas. Grouping and ranging patterns appear to be related to foraging strategies in different ways in each sex. Females often joined parties for a short time only (< 1 1/2 h), apparently at rich food sources. Males tended to stay for longer, even though they then spent less time feeding than when alone. Spending all day in a party was associated with reduced feeding time for both sexes, but on these days ranging patterns differed between the sexes because males, but not females, travelled further when in parties. The results support the idea that the form of the chimpanzee social system is determined by the interaction of two different strategies: females attempt to forage so as to maximize net energy intake, while males sacrifice an optimal foraging strategy for the sake of reproductive competition.