Along the 500 km of the Catalan coast (NE Spain), from the littoral fringe to the continental slope, there is a considerable heterogeneity of substrat and benthic communities. Cnidarians have been collected from all these communities in surveys that followed the three axes of maximum variance in the zone: bathymetric, seasonal, and geographic. The sampling surveys were of three kinds: a) Coastal sub-littoral surveys, at six stations along the Catalan coast; b) Coastal sub-littoral surveys centered on a specific locality; c) Shelf surveys, along the continental shelf. The results obtained show that in the shallower sub-littoral communities, in environmental conditions which are more dynamic and unstable, short-lived species (Hydrozoa) characterized by stolonial growth, small average biomass, and small polyp size predominate. On deep, hard bottoms, larger, massive of erect (mainly Anthozoa) species, with longevities of from one to a few years, and larger polyp size predominate. ON deep, soft bottoms on the continental shelf these massive encrusting and erect sessile species are replaced by mobile and large ones. These general trends are occasionally modified by species well adapted to specific conditions. Such is the case with certain species of Anthozoa, which can develop in fluctuating emvironments with strong hydrodynamism, and with some species of Hydrozoa which, by virtue of their opportunism, are able to seasonally colonise rocky bottoms or sandy depths. These exceptions to the general rule confirm the adaptive ability of the Cnidarians, which are good indicators both of biotic conditions and of different habitats in the marine environment.