Algal growth potential was used to quantify the degree of eutrophication of three coastal regions in São Paulo, Brazil which are subject to sewage disposal. Surface water was collected in Praia Grande, Santos, Guarujá and São Sebastião during two surveys (low tourist season—October 1997 and high tourist season—March 1998). Water was filtered and used in chlorophyll-a and nutrient analyses and in bioassays. Samples were inoculated with the diatom Phaeodactylum tricornutum Bohlin, and then subdivided into seven replicates. During a ten days experiment, in vivo fluorescence was performed and algal growth potential was calculated. Values were expressed as absolute in vivo fluorescence, as well as with a ratio of in vivo fluorescence of sample in relation to the fluorescence of control sea water for each treatment which defines the trophic status. The results of these bioassays showed a growing degree of eutrophication among regions: Praia Grande ∼ São Sebastião → Guarujá → Santos. It also identified sewage disposals and estuarine discharges as the main sources causing eutrophication. This bioassay technique is an important tool for environmental pre-monitoring studies, giving useful results in a short time.