The metabolism of the insecticide momfluorothrin (1), 2,3,5,6-tetrafluoro-4-(methoxymethyl)­benzyl (EZ)-(1R,3R)-3-(2-cyanoprop-1-enyl)-2,2-dimethylcyclopropanecarboxylate, 14C-labeled at the benzyl or cyclopropyl carbon, was investigated in lettuce. The metabolic profiles were similar between the two active ingredients, 1-R-trans-Z and 1-R-trans-E. On the leaf surface, 1 gradually volatilized and penetrated into the plant with concomitant degradation to form aldehyde/carboxylic acid derivatives via oxidative cleavage of the propenyl double bond. No isomerization of 1 proceeded at any chiral carbon. In the leaf tissues, 1 underwent ester hydrolysis to give the corresponding alcohol and chrysanthemic acid moieties, followed by glucose conjugation and successive malonic acid or ribose modification. Assuming O3 or 1O2 as the major reactant for the degradation on the plant, the reactivity with the alkenyl group in the substructure methyl (1R,3R)-3-[(Z)-2-cyanoprop-1-enyl]-2,2-dimethylcyclopropane-1-carboxylate was estimated from the HOMO/LUMO energy at the B3LYP/6-311+G** level, which indicated a lower potential of 1 than analogous pyrethroids due to its electron-withdrawing cyano group.