The phenylurea herbicide isoproturon, 3-(4-isopropylphenyl)1-1-dimethylurea (IPU), is widely used to control pre- and post-emergence of grass weeds and broad-leaved weeds in cereal crops. It is also occasionally found in ground, drainage and surface waters. Degradation in soils is a key process for assessing the risk of contamination of water resources by agricultural pesticides. The degradation properties of various samples from surface (plough layer) and subsurface soil (down to 1 m depth) of a heterogeneous field in the Beauce agricultural region of France were studied using 14C-IPU. Laboratory incubations were carried out at 22 and 10°C and at water contents representing 90 and 50% of the soil water holding capacity (WHC). Degradation was found to be more sensitive to water content variations than to temperature variations in the ranges that we used. For surface layers, at 10 and 22°C, the degradation half-life increased by a factor 10 and 15, respectively, when water content decreased from 90 to 50% WHC. Under optimal degradation conditions (i.e. 22°C and 90%WHC), monodemethyl-isoproturon was the main metabolite in surface samples. In subsurface samples, IPU half-lives were larger than 100 d, IPU was the main identified compound after 92 d of incubation and the main metabolite was an unidentified polar metabolite. These results suggest an alternative metabolic pathway probably involving hydroxylations in subsurface materials. Clay content seemed to play a major role in IPU degradation in both surface and subsurface horizons by decreasing IPU availability in soil solution and/or by limiting water availability for microorganisms.