ABSTRACT:The fish communities in three sewage receiving Illinois streams were monitored by electrofishing as treatment changed from secondary with chlorination to secondary without chlorination to tertiary without chlorination over a 40‐month period. Degradation of the fish communities was severe during the secondary treatment/chlorination period. When treatment was changed to secondary without chlorination, strong improvements occurred in the fish communities of all streams, and degradation could no longer be demonstrated on the basis of species number. The onset of tertiary treatment resulted in few additional changes in the fish communities, with the possible exception of increased abundance of several species of minnows. Tertiary treatment consisted of nitrification, sand filtration, and, in two streams, phosphorus remova