Exploration of the northern Peruvian subduction zone with the French submersible 'Nautile' has revealed benthic communities dominated by new species of vesicomyid bivalves (Calyptogena spp. and Vesicomya sp.) sustained by methane-rich fluid expulsion all along the continental margin, between depths of 5140 and 2630 m. Videoscopic studies of 25 dives ('Nautiperc' cruise 1991) allowed us to describe the distribution of these biological communities at different spatial scales. At large scale, the communities are associated with fluid expulsion along the major tectonic features (scarps, canyons) of the margin. At a smaller scale on the scarps, the distribution of the communities appears to be controlled by fluid expulsion along local fracturation features such as joints, faults and small-scale scars. Eight dives were made at one particular geological structure, the Middle Slope Scarp (the scar of a large debris avalanche) where numerous clam beds have been discovered. The spatial distribution of the chemosynthetic communities on this scarp indicates extensive fluid expulsion, but the low clam densities and low faunal diversity in clam beds and the presence of dead beds suggest fluid expulsion is temporally irregular. An exceptionally large clam field was observed at the bend of this scarp at the intersection of 2 faults. It extends for about 1000 m(2) with the biomass of bivalves as high as 30 kg m(-2) wet weight without shells. A strong and regular fluid flow, estimated at 400 m yr(-1), is required at this location to generate such a high chemosynthetic primary production. Temperature anomalies in the sediment were measured in situ in the main field and compared to clam density. The distribution of the bivalves at the metre scale is likely related to local variations in fluid flow and in fluid expulsion patterns controlled by the nature of the subsurface sediment. Exceptionally large and densely distributed serpulids (Neovermilia n. sp.) were arranged in clumps of 20 to 30 m(2), and covered 200 m(2) of the field. Their abundance may be related to the filtration of chemoautotrophic free-living bacteria. Another 22 non-symbiotic species displaying various trophic strategies have been sampled or observed, and 5 are still undescribed new species. The resulting community structure is more complex than in other subduction systems described so far.