Fenitrothion was applied to two small streams to observe the drifting patterns of invertebrates. A wide variation in the drifting patterns was found. Baetis sp. and Nemoura sp. drifted rapidly, but Elmis sp. and Ephemera japonica began to drift very late. Simulium could be classified as belonging to a rather early drifting group. The natural drifting from the upper stream did not seem to contribute to the recovery of fauna. The dominant species in the course of recovery changed from month to month. Simulium increased in density to a much higher level than in the previous two months after insecticide application. Anisogammarus sp., which habited the streams in large numbers, disappeared and did not recover even after four months even though the upper regions of one of the streams studied remained free of the chemical.