Two synthetic pyrethroids, cypermethrin and fenvalerate, were assessed for their effects on soil algae. Single or two repeated applications of cypermethrin or a single application of fenvalerate to soil at 0.5 to 1.0 kg ha-1 had no inhibitory effect on algal flora. Two applications of fenvalerate, at concentrations of 0.75 to 5 kg ha-1, resulted in a significant increase in populations of algae. No change in the population size was observed even when either monocrotophos or quinalphos was added between two applications of a pyrethroid, all at the 1.0 kg ha-1 level.