ABSTRACT. Composition and distribution ofCrustacea (Decapoda) from Lagoa do Peixe, Rio Grande do Sul, Brazil. The Lagoa do Peixe ("Peixe's Lagoon") is located between Atlantic Ocean and Lagoa dos Patos and is 30 Km long and 0,5-2,0 Km wide (31°13 ' S, 50 0 55'W -3 1 0 26'S, 51009' ). For one year, the composition and distribution of Crustacea Decapoda were analyzed monthly (fTOm Julyl1 994 to June/1995), in seven subareas (terrestrial border and aquatic areal and physical and chemical parameters (depth, salinity, sediment texture, oxygen in water, organic material of sediment and water temperature) were registered . Decapoda fTOm the families Grapsidae, Ocypodidae, Portunidae (Brachyura), Diogenidae (Anomura) and Penaeidae (Penaeidea) were captured. Chasmagnathus granulata Dana 1851 was found ali months near the canal connecting A tlantic Ocean and Lagoa do Peixe, but inside the lagoon it was collected only in October/94 and June/95, and in extreme subareas it was sampled in February/95 when the salinity was 31 %0 at these sites. Callinectes sapidus Rathbun, 1896 and Cyrtograpsus angulatus Dana, 1851 were captured throughout the year, except October/94 and April/95, respectively. Both species were found in ali areas of sample, indicating a high plasticity in relation to the factors analyzed in this study. Another species captured probably come in the lagoon in specific months, when the environrnent provides ideal conditions for their developments.