Abstract.1. Evidence used to support the theory that mixed function oxidases (MFOs) are important in the survival of polyphagous herbivores is briefly reviewed. This evidence includes data on patterns of variation in MFO activity among species and among developmental stages within species. It also includes data on induction of MFO activity by plant compounds and metabolism of plant cornpounds by MFOs.2. It is argued that the evidence presently available does not offer strong support of the theory because key pieces of information are lacking. Evidence which tends to refute the theory is reviewed and discussed.3. Experiments are proposed which could more rigorously test the theory.