The photolysis of the fungicide metalaxyl in aqueous solution has been examined, Irradiation at 254 and 290 nm resulted, respectively, in 53 and 10% substrate transformation in 3 h. Long-time (65 h) irradiation under artificial sunlight in the presence of commercially available humic acid resulted in 65% degradation of the chemical. The photolysis leads not only to rearrangement of the N-acyl group to the aromatic ring but also to demethoxyiation, N-deacylation, and elimination of the methoxycarbonyl group from the molecule.