1. Immature animals from three social groups of vervet monkeys (Cercopithecus aethiops) living in adjacent territories were observed in Amboseli National Park, Kenya. Measures of diets and food densities suggested that food quality and distribution differed both between the groups and between the dry and wet seasons, such that the dry season was a period of low food quality, and Group A had the lowest food density. 2. Seasonal differences in dietary quality and the time spent feeding produced seasonal differences in the rates of high energy social interactions: play, aggression, and competition. 3. Differences between groups in food availability and quality affected rates of social interactions such that Group A generally had the lowest frequency of interactions. Both between groups and between seasons, low food quality reduced the frequency of interactions and increased the time spent feeding. 4. Play rates were shown to be highly responsive to the energy estimated to be available from the diet during the different seasons such that play was eliminated from the immature social repertoire during the stressful dry season. 5. The time course of weaning also appeared to be influenced by seasonal and inter-group differences in habitat. Few females gave birth in consecutive years, and many suckled their infants into the second year of life. Stressed females weaned infants earlier. It is suggested that prolonged investment is a response to environmental variation and plays an important role in female reproductive strategies. 6. It is suggested that the immatures in these groups maximised the time and energy available for different types of social interactions in a different way in each season and in the different groups. This social flexibility may be viewed as part of an adaptive complex of behaviour during the immature stage of a life history.