Acute and chronic toxicity of high-grade chlordane (98%) and bioaccumulation were investigated in Daphnia magna at water soluble concentrations obtained without cosolvent. The measured effective concentrations immobilizing 50% of the microcrustacea (95% confidence interval) were 22.6 (19.7-26.1) μg/L at 24 h and 13.4 (11.3-15.8) μg/L at 48 h. This indicated an increase of chlordane toxicity with time of exposure as confirmed in chronic studies. After 21 d of exposure, significant effects on survival were recorded at a chlordane concentration greater than 2.9 μg/L, whereas reproduction (number of offspring per adult, brood size) and length of adults decreased at 0.7 μg/L or more in a concentration- and time-dependent manner. The production of male offspring and developmental abnormalities, consisting of underdeveloped second antennae and shell spines in live neonates, were recorded. The chlordane concentration tested with no significant adverse effect (NOEC) on reproduction of daphnids after 21 d compared with controls was 0.18 μg/L. The bioaccumulation factor of chlordane by daphnids exposed at a level of concentration close to the 21-d NOEC reached 10,600, wet weight, and 244,000, dry weight, after 40 d. The trans-chlordane bioaccumulated to a greater extent than the cis isomer in daphnids, whereas the cis isomer was predominant in the test medium. The results suggest a crucial role of the invertebrates in transfer of chlordane in aquatic food webs and can be used to derive a freshwater guideline for environmental protection accounting for bioaccumulation.