Organophosphorus insecticide-tolerance and degrading abilities of five zooplankton species, including Sinocalanus tenellus a dominant species in Lake Shinjiko and Oithona davisae a dominant species in Lake Nakanoumi, and rate constants for disappearances of insecticides in environmental water were examined. The contribution of zooplankton to disappearance of insecticides in environmental water was evaluated as the ratio of insecticide disappearance rate caused by zooplankton to the disappearance rate in lake water. The tolerance of S. tenellus and O. davisae to phenthoate and diazinon was lower than the tolerance to other tested insecticides, such as cyanophos, salithion, malathion, fenitrothion and dichlofenthion. Disappearance of malathion and dichlofenthion from sterilized lakes water and artificial brackishwater in the presence of zooplankton were faster than those of other chemicals. Malathion-degrading activity of zooplankton was high and the activity for dichlofenthion was low. The insecticide-degrading abilities depended on insecticide types. The maximum contributions of zooplankton to disappearances of several insecticides in Lake Shinjiko were estimated as follows : 0.4% for fenitrothion, 8.1% for malathion and 3.4% for salithion in spring, and 0.6% for fenitrothion, 4.6% for cyanophos and 0.8% for diazinon in autumn. The maximum contributions of zooplankton to disappearance of several insecticides in Lake Nakanoumi were estimated as follows : 0.1% for fenitrothion, 0.7% for malathion and 1.9% for salithion in spring, and 0.1% for fenitrothion, 2.7% for cyanophos and 1.2% for diazinon in autumn.