The route of degradation of diazinon in laboratory under aerobic conditions was assessed in various soil types: sandy loam, humic sandy, loam, loamy sand, clay loam and silt loam. Degradation of diazinon occurs via the oxidation of the phosphorothioate ester of the parent molecule to G 27550 (2-isopropyl-4-methyl-6-hydroxypyrimidine) with further oxidation of the isopropyl moiety to the minor metabolite GS 31144 (2-(1-hydroxy-1-methyl)-ethyl-4-methyl-6- hydroxypyrimidine). Mineralisation to carbon dioxide is likely to proceed via G 27550 and GS31144 (as observed by the lag phase in the production of the carbon dioxide) rather than from direct mineralisation of diazinon. The principal metabolite, G 27550 accounted for a maximum of 81.8% AR after 21 days incubation (20oC, 60%FC) and decresed to 0.8% AR by the end of the incubation (day 119). The minor metabolite, GS 31144, was also detected at very low levels, representing a maximum of 3.0% AR. The major degradation product was carbon dioxide, representing a maximum of 85.6% AR in one sample (119 days incubation). Organic volatile radioactivity accounted for <1% AR in the microbial viable soils (7.8% in the sterile soils) and was characterised as being unchanged diazinon. The evolution of volatile radioactivity was rapid following an initial lag phase. The rate of degradation of diazinon was also investigated under aerobic conditions with first-order DT50 values ranging from 8 to 23 days at 20oC and DT90 values ranging from 22 to 75 days at the same temperatures. DT50 and DT90 values obtained for the incubation at the lower moisture content were 8 and 44 days respectively and mineralisation to carbon dioxide was considerably lower. This result indicated that the moisture content of the soil is an important variable in the rate of aerobic degradation of diazinon. A slower decline was observed in incubations conducted at 10oC and using sterile soil (DT50 values of 12 and 118 days respectively). These results and the fact that diazinon is hydrolytically stable at neutral pH values demonstrates that degradation of diazinon in soil is mainly via a biotic mechanism. Therefore, lower temperatures and drier conditions both reduce biological activity and thus effect the rate of mineralisation.