The degradation of malathion in soil under aerobic and anaerobic conditions was examined in three studies. The soils used were a loam, a silty clay, two silty loams, a sand and a sandy loam. The primary route of degradation is microbially mediated, as an examination using sterile soils yielded no significant degradation of malathion. Under aerobic conditions, the predominant pathway of malathion degradation is hydrolysis of the ester bond to the monocarboxylic acid (max. 25%) and subsequently to the dicarboxylic acid (max. 62%). Following this malathion rapidly degrades to carbon dioxide (50-67 % AR after 92-162 days), along with bound residues although malic and lactic acid were detected, along with small amounts (< 10 %) of glycolic acid, succinic acid and tartaric acid. In one study low levels (< 10 %) of methylated esters of the monocarboxylic acids and dicarboxylic acids were formed but these were subsequently found to be artefacts of the extraction procedure. Malaoxon was detected in one study at trace levels, however it was at its maximum level (1%) at 0 hours, indicating it was probably introduced as a contaminant in the radiolabelled material applied to the soil. Under anaerobic conditions, malathion in soil follows essentially the same degradation pathway. Malathion on the soil surface is insensitive to photodegradation.