The life cycle of the damselfly Calopteryx atrata was investigated in relation to pesticide contamination occurring in its aquatic habitat. Calopteryx atrata emerged from the River Onogawa around May and stayed as immature adults in forests away from the stream. From late June to mid-August, mature adults were engaged in reproduction at the stream. On the other hand, pesticide contamination occurred from April to August with its peak in May and June, following transplantation of young rice plants. Mature nymphs of C. atrata experienced pesticide contamination, but may have tolerated it. Hatched nymphs had high susceptibility to two of the commonly used insecticides, fenitrothion (mortality occurred at > 4.0 p.g 1 1 in 24 h and at > 2.0 lag 1-1 in 48 h) and fenthion (> 2.0 lag 1 1 in 24 h and > 1.0 lag 1-1 in 48 h). Hatching was estimated to occur mainly in August, when pesticide contamination was not as high as the susceptibility level. However, the level of pesticide contamination in August is variable due to its origin from aerial spraying, so hatched nymphs may experience a hazardous amount of pesticides depending on the year or place. The population of C. atrata does not escape the risk of pesticide contamination completely and may be affected by it.