Standardized measures of behavior can be powerful tools for predicting effects of human activities on natural populations of mammals. We quantified the diurnal activity budget of dusky dolphins (Lagenorhynchus obscurus) in Golfo Nuevo, Argentina, by examining variation in activity as a function of season and age composition of social groups. Observations were made from a research vessel during summer and autumn from 2001 to 2005. Focal group-follow methodology was used. The predominant activity in each social group was recorded using instantaneous sampling, with a 2-min interscan interval. The main daytime activity of dusky dolphins was traveling, followed by milling and feeding. Mother and calf pairs spent more time milling and resting, whereas larger groups of adults and juveniles as well as mixed–age-class groups spent more time traveling and feeding. Although a seasonal pattern of variation in group size and composition was found, little seasonal variation was found in activity budgets, which were almost constant during daylight hours. The activity budget generated by this study provides a baseline for detection of behavioral differences associated with tourism and other human activity in the region.