Data collected south-west of the Balearic Islands (Western Mediterranean) on two cruises, Quimera-I (October, 1996) and Quimera-II (May, 1998), were used to establish general ecological patterns in deep-sea fish assemblages. A total of 39 trawls were taken at depths between 400 and 1714 m. Fish assemblages were analysed in terms of species composition, ecological parameters and biomass spectra. Differences in species composition between cruises and depth (Upper slope: 400-800 m, Middle slope: 801-1400 m and Lower slope: 1401-1714 m depth) were evaluated using multivariariate analysis. The most important feature was the existence of a biomass peak at intermediate depths. Moreover, the analysis of the biomass spectra indicated the absence of large individuals at the shallower depth interval considered. To determine the causes of the biomass peak on the middle slope four hypothesis were explored: (1) species richness is higher on the middle slope; (2) the fish assemblage from the middle slope can be characterised by the highest abundance (number of fish/area); (3) species found on the middle slope are larger (the species assemblage is built up by larger species); and (4) there is a trend for the specimens of several species to be larger at the middle slope. The data gathered support that the biomass peak is better explained by the existence of abundance peaks of medium and large-sized species, and by the presence of some of the largest individuals of other large-sized species, in both cases related to specific depth size trends which do not always follow a bigger-deeper trend.