The ichthyofauna of the Plata River Basin (Argentina, Uruguay, Paraguay, Brazil, Bolivia) consists mainly of illiophagous (mud-eating) and detritivorous species. The primary productivity of phytoplankton is generally low. Regressions of ichthyomass and catch per unit effort against total organic nitrogen, total organic carbon, and other variables indicate that much of the spatial variability in fish abundance is explained by the content of the organic matter in the water column. These relationships are demonstrated for the Middle Paraná River and for the Salto Grande Reservoir on the Uruguay River. More limited evidence suggests that water column organic matter influences fish distribution elsewhere in the basin as well, and may account for the higher average fish abundance at the mouths of tributary rivers and streams of the Paraná and Uruguay rivers.