Mixed-function oxidase availability in the detoxification of rotenone by the herbivorous spider mite,Tetranychus urticae, and its predator mite,Amblyseius fallacis, was estimated by synergism with piperonyl butoxide. Oxidase availability was lower when the mites were reared on cucumbers than when they were reared on lima bean varieties. This was not due to the presence of cucurbitacins, because there were no significant differences in LC50's between mites reared on bitter and nonbitter cucumber varieties. The herbivore was not significantly more affected by plant species than the predator.