The aerobic route and rate of degradation of phenyl-UL-14C and 3,5-triazole-14C labelled prothioconazole under dark conditions was investigated in two laboratory studies. The first study used a German sandy loam soil (Laacher Hof) and a USA silty clay loam soil (Stanley). Total recovery of radioactivity was >90% AR. CO2 was produced at up to a maximum of 5.9% AR at 90 days. Other volatiles accounted for <0.1% AR. Unextracted radioactivity reached a maximum of 46.2% AR at 120 days. The only major metabolite (i.e. > 10% AR) was prothioconazole-desthio (M04), detected at maximum levels of 20.9% to 42.3% AR. No other major metabolites were detected, although six minor metabolites were detected at levels in the range <0.1 to 5.5% AR. Degradation of the active substance was relatively rapid, with levels of 3.1 and 10.5% AR being detected on day 120 in soils Laacher Hof and Stanley, respectively. The second study used a German silt soil (Höfchen) and a USA loamy sand (Byromville). The study was replicated using both phenyl-UL-14C-labelled and 3,5- triazole-14C-labelled prothioconazole. Total recovery of radioactivity was in the range >90% AR. In the Höfchen soil, CO2 accounted for 10.7% AR from the phenyl-label and 2.0% AR from the triazole-label at 120 days, demonstrating differences in mineralisation from the different radiolabels. In the Byromville soil CO2 levels accounted for 3.0% AR and 0.3% AR at 120 days from the phenyl- and triazole-labels, respectively. Other volatiles did not exceed 0.1% AR at any sampling interval. Unextracted radioactivity reached a maximum of 42% AR (phenyl-label), 78.3% AR (triazole-label) at 120 days in the Höfchen soil; and 40.1 % AR (phenyl-label), 42.6% AR (triazole-label) in the Byromville soil. The major metabolite prothioconazole-desthio reached a maximum level of 46.5% AR at 7 days in the phenyl-labelled Höfchen soil and 49.4% AR at 7 days in the triazolelabelled Höfchen soil. The corresponding values in the Byromville soil were 41.2% AR at 90 days (phenyl-label) and 38.4% AR at 90 days (triazole-label). Another major metabolite (i.e. > 10% AR) named prothioconazole-S-methyl (M01) was detected with maximum levels as follows. Höfchen soil: phenyl-label, 11.3% AR at day 1; triazolelabel, 12.8% AR at day 1. Byromville soil: phenyl-label, 13.7% AR at day 7; triazolelabel, 14.6% AR at day 7. No other major metabolites were detected, although six minor metabolites were detected at levels in the range <2.0 to 3.7% AR during the first 120 days of the study, with levels of one metabolite (prothioconazole-sulfonic acid, M02) reaching 8.3% at 181 days after treatment. Degradation of the active substance was relatively rapid, with <2.0% and 5.6% AR (phenyl- and triazole- labels) and 2.5%, and 10.8% AR (phenyl- and triazole- labels) being detected on day 120 in soils Höfchen and Byromville, respectively.