Chronic effect and no effect concentrations (28 day) and acute toxicity (48 hr, LC50 and EC50) values were determined forDaphnia magna with some chlorinated benzenes, chlorinated ethanes, and tetrachloroethylene. Acute and chronic toxicity generally increased with the degree of chlorine substitution with these chemicals. The 48 hr LC50 values for hexachloroethane, pentachloroethane, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, 1,2-dichloroethane, 1,2,4-trichlorobenzene, 1,3-dichlorobenzene, and tetrachloroethylene were 2.9, 7.3, 62, 190, 270, 2.1, 7.4, and 18 mg/L, respectively. The lowest observable effect concentrations (LOEC) based on either reproductive impairment or growth for 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, 1,2-dichloroethane, 1,2,4-trichlorobenzene, 1,3-dichlorobenzene, and tetrachloroethylene were 14, 26, 20, 0.69, 1.5, and 1.1 mg/L, respectively. Length of animals at the end of chronic exposure was at least as sensitive a measure of toxic effect as reproduction with every chemical tested except 1,2-dichloroethane. The acute-chronic ratio ranged from 3 to 16 with these chemicals.