The effects of two pyrethroid insecticides, cypermethrin and fenvalerate, on a green alga (Scenedesmus bijugatus) and three species of cyanobacteria (Synechococcus elongatus, Nostoc linckia, and Phormidium tenue), all isolated from a black cotton soil, were studied using either cell number or chlorophyll a as toxicity criterion. All the four species were either unaffected or stimulated at 5 micrograms ml-1. Of the two insecticides, cypermethrin, at 10 to 50 micrograms ml-1, inhibited S. bijugatus while these concentrations stimulated or only slightly inhibited the growth of S. elongatus. There was a significant inhibition in the growth of S. bijugatus and stimulation in S. elongatus with 10 to 50 micrograms ml-1 fenvalerate. The growth of N. linckia was enhanced by both insecticides while P. tenue was significantly affected.