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Mollusc species composition, abundance and spatial distribution were analysed from April 1996 to October 1997 on two tidal flats in Paranaguá Bay (SE Brazil) at the entrance (St. I) (Lat. 25o3´S) and in the intermediate sector (St. II) (Lat. 25o3´S) of the estuary. At St. I, with the highest and least variable salinity, species richness (R = 40) and diversity (H´= 1; DS = 0.85) were higher than at St. II (R = 22 and H´ = 0.54; DS = 0.57). The tropical clam Anomalocardia brasiliana was the most conspicuous mollusc of the two assemblages, followed by Cerithium atratum, Neritina virginea and Crassostrea sp. at St. I and by Macoma constricta and N. virginea at St. II. On the two flats, the decrease in species richness and abundance towards the upper tidal levels suggests a strong relationship between relative tolerance to physiological stress in the organisms and the degree of immersion/submersion.