Three sites on two rivers in Michigan were chosen to represent a gradient of eutrophication. Brown trout (Salmo trutta) were found in each site and 1,361 specimens were collected by electrofishing between March and November 1970. In the same period, 644 specimens of mottled sculpin (Cottus bairdi) were collected in the most eutrophic sites, and 556 specimens of slimy sculpin (Cottus cognatus) were collected in the two least eutrophic sites. In streams common to both sculpin species, mottled sculpin occurred in a relative abundance ratio of 10:1 with slimy sculpin. Brown trout were aged by scale annuli interpretation, and sculpin were aged by otolith analysis. Growth curves constructed for the seven fish populations indicated that for every species, fish of comparable age were larger in the more eutrophic streams. Instantaneous growth rates for the populations in each species differed only in the first year, with higher rates being exhibited by the populations in the most eutrophic environments.