Benthic macro fauna (Decapoda) was sampled by beam trawl at 6 stations in Bay of Fortaleza and adjacents Escuro e Claro rivers (São Paulo, Ubatuba, Brasil). Samples were taken monthly during 1973-1974. Diversity was high in muddy sand sites of Bay of Fortaleza and low at very shallow stations located in the rivers, where the effects of extreme fluctuations of salinity were evident. The changes in species composition appeared to correspond most clearly to differences in salinity. The total number of species of decapods collected in the areas was 27 (14 species of Natantia and 13 of Reptantia). Juveniles of Penaeus brasiiiensis and Penaeus paulensis reach the rivers. Callinectes danae live in the Bay and in the rivers. Xiphopeneus kroyeri is a specie who inhabit the Bay of Fortaleza in high populational density.