The aim of the study was to determine the impact of the increase in the organic matter content in the bottom sediments on the benthic fauna structure. The research was carried out in a strongly rheolimnic the Wloclawek Dam Reservoir (Poland). The samples were taken on eight dates from March 1988 to February 1989. The sampling sites were located at the following depths: 1 m, 2 m, 5 m, 8 m and 10 m. The organic matter content in the bottom sediments increased with depth from 0.6% at a depth of 1 m to 13.2% at a 10 m depth. To a depth of 8 m, the bottom fauna was relatively rich in quality. The significant decline in the diversity and, above all, the number of taxa of the macrozoobenthos groups, only at a depth of 10 m were found. In turn, the abundance (density and biomass) of the bottom fauna increased with depth of the site. The analysis of the increase in Oligochaeta abundance with depth suggests that at the level of about 4% of the organic matter content in the bottom sediments, the food conditions are no longer a factor limiting the Oligochaeta occurrence.