The effects of shoreline changes on coastal habitats, such as loss of faunal biodiversity, remain as central topics of marine ecological research. The distribution of marine invertebrates in the beaches is partly directed by the composition of the substrates. However, the relationship between sand composition and mollusk diversity has not been assessed yet in the beaches of Puerto Rico. In this study we determined patterns of abundance and species richness of both gastropods and bivalves, in which there were collected within transects, in four selected beaches on the western coast of Puerto Rico: Balneario Tres Hermanos (Añasco), Córcega (Rincón), El Combate (Cabo Rojo) and El Maní (Mayagüez). Our hypotheses were that the patterns of abundance and species richness of both gastropods and bivalves varied significantly among the selected beaches, and mollusk diversity was somehow correlated to sand composition. Every locality showed a distinctive mollusk fauna. However, there were no correlations between diversity and substrate composition. This study will serve as baseline characterization for future studies, for monitoring and management programs for the knowledge and preservation of biodiversity of shell-mollusks found at the Caribbean.