Detoxification enzymes were compared in 36 arthropod species representing both chewing and sucking herbivores and their natural enemies. Enzymes studied include aldrin epoxidase (MFO), trans-epoxide hydrolase (trans-EH), cis-epoxide hydrolase (cis-EH), and l-naphthyl acetate esterase. Major selectivities were found for MFO and EH. High MFO and trans-EH activities were consistently associated with herbivory, whereas entomophagous arthropods had a low trans-EH to cis-EH ratio. Phloem-sucking insects were different, exhibiting a low trans-EH to cis-EH ratio.