We sampled the communities of decapod crustaceans inhabiting the depth zone between 3 and 871 m off the Catalan coast (North-West Mediterranean) from June 1981 to June 1983. The 185 samples comprised 90 species differing widely in their depth distributions. Multivariate analysis revealed four distinct faunistic assemblages, (1) littoral communities over sandy bottoms, (2) shelf communities over terrigenous muds, (3) upper-slope communities, and (4) lower-slope or bathyal communities. The brachyuran crab Liocarcinus depurator is the most abundant species of the shelf assemblage, although L. vernalis dominates over the shallow sandy bottoms of the shelf. The dominant species of the upper-slope assemblage are nektobenthic species (Solenocera mernbranacea, Plesionika heterocarpus, Processa canaliculata), pelagic species (Pasiphaea sivado, Sergestes arcticus), and benthic species (Macropipus tuberculatus, Munida intermedia, Nephrops norvegicus). A risteus antennatus comprise most of the biomass of the lower-slope community, which supports a greater diversity than the other assemblages. The main assemblages appear to be related to different hydrological characteristics, the extent of seasonal fluctuations, and to the changes in sediment structure associated with changes in the steepness of the bottom.