The route and rate of aerobic degradation of aminopyralid has been determined in four European soils under laboratory conditions in the dark at 20oC (for one soil also at 10oC and 30oC) and 40% moisture holding capacity. Aminopyralid was steadily degraded in soil under aerobic conditions. The only metabolite observed was CO2 indicating that the phenyl ring of aminopyralid is mineralised. No other degradation products were detected. Little or no mineralisation was observed under sterile conditions, demonstrating that the degradation was microbial. After 92 days at 20°C, the CO2 accounted for 24.1-69.3% AR, whilst the unextracted radioactivity was accounted for 10.3-21.6% AR.