Fenamiphos is an important nematicide-insecticide and commonly used in horticultural crops and turfs in Australia. We studied the transformation/degradation of fenamiphos under controlled conditions, in surface and subsurface soils from the Swan Coastal Plain of Western Australia. In the sandy surface soil, fenamiphos (F-en) was rapidly oxidised to its sulfoxide (FenSO) analogue. Further oxidation of FenSO to sulfone (FenSO2), however, was found to be very slow, resulting in an accumulation of FenSO. Little accumulation of FenSO2 occurred during the study period (139 days). The time taken for 50% loss of the total residue of fenamiphos (F-en+FenSO+FenSO2) was found to be approximately 50 days in the surface soil and about 140 days in the subsurface soil. Simulations using the LEACHM model showed that the rate of transformation of F-en to FenSO in the surface soil (k(1) = 0.5/day) was 100 times faster than that of FenSO to FenSO2 (k(2) = 0.005/day). In the subsurface soil, the difference between the 2 oxidation steps was much smaller (4-fold). The conversion of F-en to FenSO was much faster in the surface soil (k(1) = 0.5/day) than the subsurface soil (k(1) = 0.02/day). The observed differences in transformation behaviour of F-en between the surface and subsurface layers of soil appear to be associated with the differences in the microbial biomass and the organic matter contents of the soils. The slower transformation of F-en in the subsurface soils can have major implications on its potential for groundwater contamination in vulnerable areas, such as the Swan Coastal Plain of Western Australia.