Data are presented on macrobenthic (;;:: 1 mm) biomass and species composition, sulfur bacteria (Thioploca), demersal fish catches, organic content of sediment and dissolved oxygen at 65 stations (35-360 m) along the upwelling area off north Peru in 1980-1981. Oxygen concentration close to the bottom was high only down to about 20 m depth and at 20-700 m it was generally < 0.8 mil-I. Organic content of sediment increased significantly with water depth.Macrofauna were found at all stations with a general dominance of small polychaetes. Macrofaunal biomass showed a significant positive correlation with oxygen concentration; below 0.6 ml 1-1 of oxygen biomass was impoverished. No correlation was found between biomass and depth. A mean macrofaunal biomass of 5.9 g 0.1 m-l was recorded at depths < 100 m and 3.1 g 0.1 m-ll at 100-200 m. Biomass was higher in the north compared with the south and showed a significant positive correlation with demersal fish catches. In contrast, Thioploca biomass showed a significant negative correlation both with macrobenthic biomass and demersal fish catches. From this study and previous work, we conclude that oxygen concentration was the dominant ecological factor determining macrobenthic biomass and species composition in the upwelling area off Peru and northern Chile. The benthic fauna living in low oxygen concentrations have probably developed this tolerance through evolutionary adaptation. Based on oxygen concentration and exposure, five zones in this upwelling area are characterized and their dominant benthic macrofauna documented.