Guiana dolphin (Sotalia guianensis) encounters were documented in Sepetiba Bay. For the purpose of this study all dolphin sightings were classified as groups or aggregations. Groups (N ¼ 268, x ¼ 20, range ¼ 1-90) consisted of dolphins in apparent association usually engaged in the same behaviour and moving in the same direction. Aggregations (N ¼ 108, x ¼ 204, range ¼ 100 -450) consisted of temporal association of numerous groups gathered in an area, totalling several hundred individuals usually engaged in the same behaviour but not necessarily moving in the same direction. Guiana dolphin groups and aggregations differed significantly in behavioural activity, spatial and temporal distribution and age composition. Most aggregated dolphins were engaged in foraging/feeding activities and primarily located at the interior of the bay, and also, aggregations were more likely recorded during morning hours and had more neonates and calves present when compared to groups. The plotted GPS records of aggregations formed two distinct clusters of distribution within the bay, indicating potential areas where conservation strategies could be implemented.