The toxicities of five synthetic pyrethroids, resmethrin, bioresmethrin, tetramethrin, allethrin, and bioallethrin, have been compared with that of natural pyrethrins against susceptible Tribolium castaneum and susceptible and pyrethrin-resistant Sitophilus granarius, using topical application techniques. When the synthetic toxicants were used alone against the three strains of insect, bioresmethrin was by far the most toxic compound, followed in order by resmethrin, bioallethrin, allethrin, and tetramethrin. Depending on the strain tested, bioresmethrin had up to 16 times more toxicity and tetramethrin as little as 1/100 the toxicity of pyrethrins. Piperonyl butoxide (PB) synergized all the compounds against the three strains. When synergized, bioresmethrin was again the most toxic of the synthetic compounds, followed by resmethrin, bioallethrin, and allethrin. Tetramethrin had the lowest toxicity against T. castaneum, but against both strains of S. granarius tetramethrin plus PB became the third most toxic of the six compounds tested. There was a smaller range of effectiveness among the compounds when they were synergized. Except for tetramethrin, factors of synergism were low against T. castaneum (1 to 4); higher against susceptible S. granarius (4 to 37); and very high against resistant S. granarius (31 to 209). Very high factors of synergism were obtained with tetramethrin (43 to ⪢ 216) against all three strains. Against the weevils, toxicant: synergist ratios higher than 1:10 gave higher factors of synergism with resmethrin and bioresmethrin; but not with pyrethrins. The influence of solvents and methods of application on LD50 measurements, was studied in tests with pyrethrins. The non-insecticidal fraction of pyrethrum extract was found to be toxic, at high concentrations, to resistant S. granarius.