Effect of chlorinated municipal wastes on fish life and laboratory findings of earlier researchers were assessed in separate studies at four Michigan municipal wastewater treatment plants. Tolerance levels and river length below each plant outfall rendered unavailable to resident fish populations were determined. Ten rainbow trout (Salmo gairdneri) and ten fathead minnows (Pimephales promelas), previously acclimated, were held 96 hours in live boxes in the stream above and below these outfalls. Fish held below these outfalls were subjected to both chlorinated and non-chlorinated exposures during effluent discharge. Test waters were monitored chemically and bacteriologically during fish exposure. Total residual chlorine concentrations below three plants were toxic to rainbow trout at distances up to 0.8 mile. Fathead minnows appeared adversely affected up to 0.6 mile downstream in two of the four plants. Total residual chlorine concentrations less than 0.1 mg/l were toxic to fathead minnows. Rainbow trout 96-hour total residual chlorine TL-50 concentration below two plants was 0.023 mg/l.