Recent extinctions of coastal dolphin species indicate that marine mammal populations are susceptible to rapid decline. Yet, effective conservation efforts depend on population-level ecological data. To obtain principal baseline data that will inform management efforts, we characterized the habitat and recorded the behaviour of a Guiana dolphin (Sotalia guianensis) population within one of the most well-preserved estuaries off southern Brazil. Monthly surveys were conducted for one year (August 1999 - July 2000) within an area of approximately 100 km2, within the Paranaguá Bay Estuary. We employed the group follow protocol, which resulted in 260 h of direct observation. Our results revealed feeding as the most frequent activity in the estuary, totaling nearly two thirds of all records. We also identified two sites of Guiana dolphin habitat preference in our study area, where sightings remarkably totalled > 62% of observation records. These sites (especially Guaraqueçaba Bay) were not only important for feeding, but also for S. guianensis socialization. The detection of these key areas should facilitate both local and broad-scale efforts to preserve critical habitats for this population of dolphins, and by extension may help inform management plans for ecologically vulnerable S. guianensis populations in other parts of their distribution.