Thirty-six boat surveys were conducted in Guanabara Bay, south-eastern Brazil, from October 2002 to June 2004 in order to investigate marine tucuxi group characteristics. The average and the range of tucuxis group size in the Guanabara Bay are similar with areas where small group sizes have been recorded. Group size ranged between one and 40 individuals (13·0±9·5; median=10·0). Groups of two to ten dolphins were the most common (53·9% of observations). Group size and the maximum number of dolphins sighted in each survey day did not vary between seasons (Kruskal–Wallis test, H3,36=3·249; P=0·355). Spatial geometry varied with group size and mixed groups were the largest (Kruskal–Wallis test, H3,218=57·149; P<0·001). The presence of calves had a great effect on group size, and nursery groups (mean =14·3±9·1; median=13·0) were twice as large than non-calf groups (mean=7·1±5·2; median=6·0). It is suggested that larger groups may aid in the calves development and learning. Group size changed frequently, resulting in 82·2% of observations with no constant size. Group size of marine tucuxi was not affected by the dolphins' behaviour (Kruskal–Wallis test, H2,215=5·626, P=0·06) neither by water depth (R2=0·012; F1,219=2·82; P=0·094).