A bacteriological and chemical study was conducted on a segment of the Trinity River, Texas, to determine the effects of chlorination of effluents. Waste effluents contributed large numbers of fecal coliforms and fecal streptococci. Chlorination reduced the quantities of microorganisms only at the stations where chlorine was directly applied, and bacterial populations recovered immediately when chlorination ceased. Beneficial heterotrophic bacteria were susceptible to chlorination. Chlorination did not seem to influence the incidence of Salmonella. Effluent BOD and DO were improved by chlorination. Chlorination did not seem to improve river conditions