We investigated the impact of fish farming on the benthic biota of 2 habitats (sea- grass meadows of Posidonia oceanica and non-vegetated soft bottoms), by examining in detail the nematode assemblages, and the effects of organic enrichment on their abundance, diversity (spe- cies richness and trophic diversity), community structure and individual size. All investigated faunal variables differed significantly between impact and control sediments, except for indi- vidual nematode biomass. Nematodes displayed reduced biodiversity beneath the investigated fish farms in both vegetated and non-vegetated habitats. The nematode genera Richtersia, Desmoscolex and Halalaimus were highly sensitive to biodeposition and disappeared almost com- pletely in farm sediments, whereas other genera such as Daptonema and Prochromadorella largely increased their relative abundance. The impact of biodeposition was investigated also in terms of K-dominance curves and functional variables (life strategies as maturity index, and func- tional [i.e. trophic] diversity) which showed that nematodes are very sensitive to this kind of envi- ronmental disturbance. Our results indicate that the analysis of nematode assemblage composi- tion, the life traits of the dominant species and the presence/disappearance of certain nematode species represent a reliable tool for monitoring the quality state of marine sediments exposed to fish-farm organic wastes.