Abstract Glutathione S -transferases (GSTs) isolated from larval midguts of the fall armyworm, Spodoptera frugi perda , were used to study their sensitivity to inhibition by 44 chemicals, most of which were plant allelochemi cals. Many flavonoids, other phenols, and α,β-unsaturated carbonyl compounds were potent inhibitors of the enzymes. For the flavonoids, apigenin was the best inhibitor among those studied, showing an I 50 value of 8.2 × 10 −7 M. In general, flavonols exhibited higher inhibitory potencies than flavones; flavonoid glycosides were less inhibitory than their corresponding aglycones. In the case of phenols, tannic acid was the best inhibitor among those tested, showing an I 50 value of 2.0 × 10 −7 M. The organotin compound chlorotriphenyltin was an potent as tannic acid in inhibiting the transferases. For the αβ-unsaturated carbonyl compounds, ethacrynic acid was the most potent inhibitor among those examined, showing an I 50 value of 1.5 × 10 −7 M. However, the isothiocyanates were found to be marginal inhibitors of GSTs. GSTs isolated from larval fat bodies showed differential sensitivity to inhibition by some selected inhibitors compared with midgut GSTs. Kinetic studies of midgut GSTs revealed that ellagic acid exhibited a noncompetitive inhibition toward 1-chloro-2, 4-dinitrobenzene (CDNB) but a competitive inhibition toward glutathione (GSH). In contrast, inhibition by tannic acid and ethacrynic acid were noncompetitive toward both CDNB and GSH. Diethyl maleate did not affect GSH levels in midguts when larvae were topically treated with this compound for up to 24 h.