The behaviour of free-ranging groups of Callithrix aurita and Callithrix flaviceps, small-bodied omnivorous marmosets, was monitored at two sites (Cunha and Caratinga, respectively) in south-eastern Brazil during equivalent periods in consecutive years. Quantitative behavioural data were collected using a standard scan-sampling protocol, with scans being taken at 5-min intervals throughout the daily activity period. Data were collected during the wet season months of January and February at Cunha in 1993 and 1994, and during the dry season months of July and August at Caratinga in 1985 and 1986. In both cases, wet-season precipitation levels were considerably higher during the first year. Activity budgets, daily ranging and the foraging behaviour of both species were compared between years. Significant between-year differences were observed in several behaviour categories in both species, and were consistent with seasonal and longitudinal variation in resource abundance. In C. aurita, apparent changes in fruiting patterns resulted in a significant increase in exudate feeding in the second year, both in relative and absolute terms, but little change in prey foraging. By contrast, a marked reduction in arthropod prey in 1986 resulted in major changes in most aspects of C. flaviceps behaviour, with the exception of plant feeding. While consistent with behaviour patterns in other marmoset species, the numerous significant differences observed in the present study further highlight the problems associated with the evaluation of intertaxon differences in the behaviour and ecology of primates, in particular marmosets, on the basis of relatively short-term studies of a few individuals, often at only a single study site.