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Toxicity of C14-ring-methyl parathion in penetration, tissue absorption, and metabolism by susceptible Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren) larvae was compared. C14 methyl parathion penetrated the cuticle of H. punctigera more rapidly than of H. armigera, leading to initially higher internal insecticide concentrations in H. punctigera. The same methyl-parathion metabolites were recovered from H. armigera and H. punctigera. However, larger quantities of methyl paraoxon and p-nitrophenol-glucoside were extracted from H. punctigera. These results are discussed in terms of the history of insecticide resistance in H. punctigera and H. armigera in Australia.