The study was conducted to determine the effects of treatment and seawater dilution of municipal wastewater on marine organisms. An experimental facility was built in southeast Florida that provided both unchlorinated and chlorinated effluent from three standard treatments: primary settling, chemical flocculation, and activated sludge secondary treatment. Exposure tests lasting longer that one month were conducted on the sheepshead minnow (Cyprinodon variegatus) and the pink shrimp (Panaeus duorarum), with each of these six effluent types at seawater dilution ratios of 30:1 100:1, and 300:1. The shrimp showed a much more sensitive response than the minnow. Almost 100% mortality occurred for shrimp exposed to the unchlorinated 30:1 seawater dilutions of primary-settled wastewater, while mortality for the other two effluents were similar to controls. Mortality could not be attributed to any of the chemicals measured in the wastewater. For the 30:1 dilution experiments, chlorination usually resulted in much higher toxicity, increasing the dilution factor from 30:1 to 100:1 reduced the mortality observed (in both unchlorinated and chlorinated tests) essentially to control levels.