Many water-soluble chlorine-containing organic compounds of low volatility have been identified in chlorinated cooling waters and sanitary effluents. Relatively little is known about the ecological effects of these materials in aquatic systems. Earlier results which indicated toxicity of two compounds, 5-chlorouracil and 4-chlororesorcinol, to carp embryos at concentrations ⪡ 1 mg/l are refuted by our research. Our investigations support more recent work indicating either relatively low or moderate toxicity (LC 50 > 10–100 mg/l) for these compounds and representative chlorinated effluents which contain them. Toxicity of three synthetic mixtures of identified chlorinated organics can be attributed primarily to the content of chlorophenols.