Species richness, abundance, and spatial distribution of the molluscan macrofauna were studied in two intertidal environments of Sao Sebastiao Channel, on the northern coast of Sao Paulo State, Brazil. The study sites, located in Sao Francisco and Engenho d'Agua beaches, were sandy substrates with rock fragments. Samples were taken in defined intertidal areas divided into three levels (lower, middle, and upper). Thirty-six 0.16 m(2) samples were collected in each level. Richness was higher at Engenho d'Agua (35 species) than at Sao Francisco (20 species). In Sao Francisco, more species and individuals were found at the lower level. The higher homogeneity in species distribution over the entire studied intertidal area in Engenho d'Agua than in Sao Francisco was probably associated with the lower slope of the former one. This shows that the occurrence and distribution of molluscs were strongly related to tidal exposure. The organic pollution in Sao Francisco also influenced the molluscan assemblage, reducing richness and diversity. (C) 2001 Academic Press.535733743