The metabolic fate of tolclofos-methyl labeled with ¹⁴C at the phenyl ring was studied in greenhouse-grown lettuce after a single application to leaves at two different rates (75 and 750 g a.i./ha). At 7 days after the application, more than half of the recovered ¹⁴C was detected as unaltered tolclofos-methyl. In the lettuce, tolclofos-methyl mainly underwent cleavage of the P-O-aryl linkage and oxidation of the aryl methyl group, followed by a rapid formation of malonylglucose and glucose conjugates, respectively. The chemical structure of the malonylglucose conjugate was determined by LC-ESI-MS and NMR analyses of the metabolite isolated from leaf discs of lettuce treated with the corresponding ¹⁴C-labeled phenol. © Pesticide Science Society of Japan