During laboratory studies of aerobic incubation at 25°C in two soils (pH 7.1-7.5, 0.52-0.58% oc), Clethodim degraded with DT50s and DT90s of 1.2-2.5 and 4.1-8.4 days (mean DT50 and DT90 recalculated to 20°C 2.8 and 9.1 days). Major metabolites (exceeding 10% AR at any time point or 5% AR at 2 consecutive time points) were clethodim sulfoxide (max. 63-73% AR on day 3-7), clethodim sulfone (max. 11-16% AR on day 30-61), clethodim oxazole sulfoxide (max. 5.2-6.0% AR on day 30-91), clethodim oxazole sulfone (max. 8.6- 10% AR on day 121-380) and an unknown compound formed from 14C-allyl-labeled clethodim (max. 6.2% AR on day 14). Besides Clethodim and the four metabolites mentioned, also clethodim oxazole (􀂔2.1% AR) and clethodim imine sulfoxide (􀂔1.6% AR) were identified. Up to 9-10 unidentified fractions were detected, the total of which never accounted for >4.5-12% AR in three experiments (including the unknown compound already mentioned which reached a maximum of 6.2% AR). DT50s for major metabolites recalculated to 20􀁱C were 27-28 days with a mean of 28 days (clethodim sulfoxide, 2 soils), 42-94 days with a mean of 68 days (clethodim sulfone, 2 soils), and 429 days (clethodim oxazole sulfoxide, 1 soil). No DT50s were determined for clethodim oxazole sulfone which showed a monotonous increase in two soils, nor for the unknown compound found at a maximum of 6.2% AR (DT50 at 25°C estimated to be between 30 and 64 days). Mineralisation of Clethodim in two sandy loam soils at 25°C was significant: 39-41% AR by day 91-99 (ring-4,6-, propyl-1 and allyl-2-14C label, 2 soils), 45-57% AR by day 121-125 (ring-4,6- and allyl-2- 14C label, 2 soils) and 55% AR by day 380 (propyl-1-14Clabel, 1 soil). In two other soils (sand and clay), mineralisation of Clethodim (allyl-2-label) at 24°C was also significant: 47-56% AR by day 28. Non-extractable residues were also formed in significant amounts but never exceeded 70% AR: for ring-4,6-label in one soil maximum 20% AR by day 60, declining to 13% AR by day 121; for propyl-1- label in one soil 18% AR by day 91, maximum 22% AR by day 183, declining to 16% AR by day 380; for allyl-2-label in one soil maximum 29% AR by day 125. Aerobic metabolism of Clethodim in soil proceeds by two parallel pathways: oxidation of the sulphur atom to give clethodim sulfoxide and clethodim sulfone, and breakdown to clethodim oxazole, which is then oxidised to clethodim oxazole sulfoxide and clethodim oxazole sulfone. The ultimate and major breakdown products are CO2 and non-extractable residues.