The toxicity of a number of topically applied pyrethroids has been tested in the laboratory against three species of locusts and parathion-resistant and susceptible strains of the Egyptian cotton leafworm. Bioresmethrin, resmethrin and 5-benzyl-3-furylmethyl (+)-cis-chrysanthemate (NRDC 119) proved to be extremely active against one or more of these pests and there were small but noteworthy improvements with the synergists sesamex and TBTP. The field potential of the pyrethroids is discussed against the background of environmental problems associated with some of the insecticides in current use.