The aim of the present study was to describe the characteristics of Guiana dolphins (S. guianensis) group size and composition in the Paranaguá Estuarine Complex (25 degrees 15'-25 degrees 36' S and 48 degrees 02'-48 degrees 45' W), Paraná State, as well as to verify the existence of relationships between a given environmental variable (water depth, salinity, transparency and temperature) and group size. An area of around 124 km(2) was surveyed by boat from April 2006 to February 2008 in the following subsets of the estuary: Canal do Superagui (approximately 28 km(2)), Pinheiros bay (approximately 34 km(2)), part of Laranjeiras bay, which included the Guaraqueçaba sub-estuary (approximately 38 km(2)), and part of the Mixture Section of the Paranaguá Estuarine Complex (approximately 24 km(2)). In 45 survey days, a total of 147 hours were spent observing 286 groups of S. guianensis. Groups varied from two individuals to aggregations of up to 90 individuals (mean +/- SD: 11.5 +/- 14.4 individuals). Solitary individuals were seen only on five occasions (1.7% of all observations). Groups with calves (n = 247) represented 86.4% overall and were more common in all sub-areas when compared to groups without calves. Groups without calves were found in all sub-areas and were usually smaller and composed of up to 12 individuals. A usual group composition of 70% of adults and 30% of calves was observed considering all sub-areas and seasons. No correlations were found between group size and water temperature (Spearman's rank test, r = 0.0059, p = 0.9338), transparency (Spearman's rank test, r = 0.0597, p = 0.9333), depth (Spearman's rank test, r = 0.1421, p = 0.4698), and salinity (Spearman's rank test, r = -0.1938, p = 0.0665). While Guiana dolphin groups were seen in the entire surveyed area, group size varied significantly among the three main surveyed sub-areas (Kruskal Wallis, H2,196 = 29.85, p = 0.0000). Groups were larger in Laranjeiras bay and smaller in Canal do Superagui. The physical environment, risk of predation, seasonal distribution and abundance of prey are the main possible factors driving larger groups in inner and protected waters.