SYNERGISM studies with the cattle tick Boophilus microplus have been effected by enclosing larvae in packets impregnated with olive oil solutions of the test chemicals and recording mortality after 48 h1,2. In these conditions larvae exposed continuously to 0.4% piperonyl butoxide were apparently unaffected. Subsequently, in studies to determine the sensitivities of the mixed function oxidase systems in organophosphorus (and carbamate) resistant and susceptible strains of B. microplus, larvae were immersed in aqueous colloids3 of piperonyl butoxide. Concentrations greater than 0.02 % proved to be toxic and an LC50 of 0.044% at 24 h after treatment was determined. This unexpected result aroused interest in the acaricidal potential of piperonyl butoxide. Although it is generally regarded as a nontoxic synergist4, some workers have claimed it is toxic to houseflies at fairly high dosage5 but others found it to be nontoxic6; toxicity to the mite Acarus siro (formerly Tyroglyphus farinae) has been recorded7,8.