Abstract: The ecological effects of BHC on Daphnia magna were tested with short-term and long-term, whole life-span, experiments. Daphnia magna, derived from a pure strain and aged6±6 hours, were exposed to 5—450 μg/1 of BHC (as γ-BHC) at 25±1℃ with 9—10 h daily lighting (3000—3300 lux). Chlorella pyrenoidosa were provided for the animals during the experiments. To assess the effects of the pesticide, ecological parameters with reference to the survival, for which death was defined as the cessation of heartbeat rather than the immobility, growth and reproduction of the animals were estimated. The innate capacity for increase (rm) of each treatment was calculated as well.The LC50 of BHC to 6±6 h old D. magna were estimated to be 2.40±0.40/μg/lfor 24 hours and 1.32±0.30μg/l for 48 hours. In long-term tests, both specific longevity.and mean body-length of the animals were found to be hardly effected by BHC if its concentration was not higher than 150 μg/l. Somewhat different responses were observed in the reproduction, on which effects of the pesticide were significant at concentrations over 200 μg/l. Considering these three criteria as a whole, the no-observed-effects concentrations (NOECs) were justified as 150 ppb and the lowest-observed-effects concentrations(LOECs) were 200ppb. The application factors were determined within the range of 0.11—0.15. As to the innate capacity for increase, significant effects of BHC were foundat 50 μg/l level and, therefore, this parameter is regarded as a more sensitive index for the toxicological assessments.