The route of degradation was studied under aerobic and anaerobic conditions using ethyl- or propyl-labelled ethoprophos. Ethoprophos was degraded in soil to the O-ethyl-S-propylphosphorothioic acid (AE 0592496) with final degradation products CO2 and unextracted residues. In one loamy sand soil the maximum level of the main metabolite O-ethyl-O-methyl- S-propylphosphorothioic acid (AE 0592496) was 7.90 % AR at 112 days declining to 2.8%AR at study end (168 days). In other soils concentrations of this metabolite were < 1% AR) at all times. Other metabolites occurred in amounts significantly below 10 % AR.