Bulb mites, Rhizoglyphus echinopus (Fumouze and Robin), were exposed for 1 h to radiocarbon-labeled samples of diazinon, formetanate, bifenthrin, DDT, chlordime-form, or bromopropylate, and the extent of metabolism was assessed at 6, 12, 24, and 48 h after exposure. Pesticide degradation by bulb mites decreased in the order diazinon > bromopropylate > chlordimeform > bifenthrin > formetanate > DDT. Based on the nature of the metabolites formed in vivo and in vitro, we concluded that bulb mites possessed active oxidases, esterases, and transferases. Bifenthrin, chlordimeform, and bromopropylate were not toxic to bulb mites (LC 50 > 1,000 ppm), but they were very active against twos potted spider mites, Tetranychus urticae Koch. The relatively fast metabolism of these three compounds in bulb mites was in contrast with their slow degradation in twos potted spider mites in other studies; probably, differential metabolism played a major role in their selective toxicity. Although formetanate was toxic to both mite species, it was much more active against twos potted spider mites. Diazinon and DDT were not toxic to bulb mites. Differential metabolism also may play a role in the selective toxicity of these three compounds, but other factors (e.g., target site insensitivity) likely are involved.