We present a quantitative analysis of the composition (commercialized catch and corresponding discard) of trawl catches, in seven ports of the western Mediterranean. Discards are defined here as the fraction of the catch from the haul that is returned to the sea by the fishermen, because it does not have commercial value. A sampling programme on board commercial was performed from June 1995 to June 1996. The hauls were sampled for the collection of the total catch in weight, by species, for the commercial and discarded fractions separately. The factors of stratification considered were in all the ports, depth, with three strata, stratum A (<150 m), B (between 150-300 m) and C (>350 m), to which factor for two types of gear in the Porto Santo Stefano or two classes of vessel power in Vilanova were added. The results obtained have allowed to characterize the commercial and discards fractions. The discarded biomass always constituted an important fraction of the total catch. However, the discard of species with high commercial interest was very low or nil