Treatment plants receiving wastewaters from domestic/commercial sources and receiving wastewater from domestic/commercial plus industrial sources were evaluated to assess toxicity reduction and determine the importance of source. Seven-day renewal fathead minnow survival and growth and Ceriodaphnia survival and reproduction tests were applied to raw wastewater, unchlorinated, chlorinated, and dechlorinated secondary effluents. All plant influents were toxic and all plant effluents showed some residual toxicity. The plant receiving the most toxic influent showed the largest reduction in toxicity and had the least toxic effluents. Toxicity at plants with little industrial wastewater flow was not reduced. Nitrification was reduced at one and a high metals concentration occurred at another plant. Toxicity reduction and industrial wasteflow did not correlate strongly and potential receiving stream impacts were noted.