The metabolism of fonofos, a thiophosphonate insecticide, was investigated in mature lettuce (Latuca sativa), beet (Beta vulgaris), and wheat (Triticum aestivum). Six new metabolites were identified by LC-MS and LC-MS-MS analysis using fast atom bombardment (FAB) and packed capillary LC columns with application of the on-column focusing technique. These methods provided the sensitivity required to identify unknown metabolites that were present in the mature plants at only 20-230 ppb. Structural elucidation was facilitated by use of fonofos labeled with both carbon-14 and carbon-13 in the phenyl ring. In all three plants fonofos was converted to a glucose conjugate of thiophenoxylactic acid. Oxidation of the glucose conjugate gave isomeric sulfoxides in all species examined. Thiophenoxylactic acid was found esterified to malonic acid in lettuce. In beets, S-phenylcysteine was found as its malonic acid amide. A second metabolite unique to beets was N-(malonyl)-[2[(ethoxyethylphosphinothionyl)oxy]phenyl]cysteine. This novel structure was confirmed by synthesis.