Mycotoxins, injected or orally ingested, adversely affect the development rate, fertility, and fitness of many insects. It is thought that the inhibition of protein translation in insect cells is among the possible reasons for such deleterious effects. Because Spodoptera littoralis (Boisduval) larvae may encounter mycotoxins in their natural habitat, we investigated the effect of three orally administered mycotoxins, aflatoxin B₁ (AFB₁), aflatoxin B₂ (AFB₂), and aflatoxin G₁ (AFG₁), on protein metabolism of the insect. The incorporation of phenylalanine into protein was investigated in penultimate and final instar larvae and pupae. There was a significant drop of phenylalanine incorporation into proteins during either the fifth larval or the larval-pupal molt. Nervous system and fat body of larvae had the highest rates of incorporation of phenylalanine into protein. Both AFB₁ and AFG₁ (at concentrations of 2.5 and 4 ppm, respectively) had profound inhibitory effects on the incorporation of phenylalanine into protein in both larvae and pupae, whereas AFB₂ (4 ppm) had only a slight effect. The typical breakdown of proteins following molting was less remarkable in AFB₁- and AFG₁-treated animals. The inhibitory effects of AFG₁ on protein metabolism in either nervous system or fat body of the last larval instar were more prominent than in alimentary canal tissues, while AFB₁ inhibited protein metabolism in all investigated tissues except hemolymph. Possible reasons for the differences in the effects of the involved aflatoxins are discussed.