The biochemical basis for the tolerance of the European corn borer,Ostrinia nubilalis, to the phototoxinα-terthienyl was investigated by measuring the midgut polysubstrate monooxygenases and glutathioneS-transferase activities.α-Terthienyl administered in the diet to the corn borers increased the level of cytochromeb 5, NADH-cytochromec reductase,O-demethylase, and glutathioneS-transferase activities. The induced detoxification enzyme activities should enable the corn borer to metabolizeα-terthienyl more efficiently and therefore render the corn borer highly tolerant toα-terthienyl.