In 1969-1972 quantitative benthos samples were collected in the rivers Aare (near Beznau) and Rhine (near Kaiseraugst). The biocoenoses are dominated by oligochaetes and chironomids in both rivers; in the Aare, Hydropsyche sp. also is one of the dominant forms. The spatial distribution of most organisms is inhomogeneous (contagious). In the Aare a significantly higher number of species, individuals, and a greater biomass have been found. As the bed of the Aare is muddy and the bed of the Rhine is gravelly and sandy, the differences in population density can be related to the differences in the substratum.