Collections of fish were made in Lytle Creek, Clinton County, Ohio, to determine the effect of the partially treated domestic sewage of Wilmington, Ohio, on the fish populations of this stream. The farther the collecting localities were downstream from the plant effluent, the greater were the numbers of species and individuals found. In the septic areas, immediately below the effluent, no fish were observed or collected. Two miles below, in the recovery zone, an average of 1.2 species and 2.2 individuals were taken per collection, with 8 different species represented. Three miles below, also in the recovery zone, 12 species were represented in our collections, with a mean of 1.7 species and 10.8 individuals per collection. In the lower portion of the recovery zone, 4.4 miles below the effluent, 32 species were taken, with a mean of 12.7 species and 197 individuals. In the clean-water zone, 36 species were taken, with a mean of 16.7 species and 294 individuals.