The oral toxicity of thePenicilliummycotoxins, alone and in combination, toHeliothis zea(Boddie) andSpodoptera frugiperda(J. E. Smith) at naturally occurring levels was evaluated. Ochratoxin A and citrinin were the most toxic alone to both insect species and caused abnormalities in the Malpighian tubules. The combination of ochratoxin A and penicillic acid was synergistically most toxic toH. zea, whereas the combination of ochratoxin A and citrinin was synergistically most toxic toS. frugiperda. The production of frequently found combinations of mycotoxins byPenicilliumfungi likely to be encountered by these insects is discussed in relation to the results obtained.