Rimsulfuron degraded rapidly in the soil under aerobic conditions, declining to less than 47 % and 52 % of applied radioactivity (AR) after 10 days in the pyridine and pyrimidine labels, respectively. The primary degradation pathway for rimsulfuron in soil is the bridge-contraction reaction to form IN-70941 and IN-70942. A secondary degradation pathway is the cleavage of sulfonylurea linkage to form INE9260 (major metabolite), and the minor metabolites IN-T5831 and IN-J290. IN-70941, IN- 70942 and IN-E9260 were identified as the principal degradation products. IN-70941 increased to a maximum amount of 54 % (pyridine label) and 55 % (pyrimidine label) AR at 2 months. The formation of IN-70942 was observed after approximately 10 days of incubation and increased to 20 % and 24 % AR after 12 months for the pyridine and pyrimidine labels, respectively. IN-E9260 (pyridine label only) reached a maximum level of approximately 16 % AR. IN-J290 and IN-T5831 (pyrimidine label only) were detected as minor metabolites (< 10 % AR).