The organophosphorus insecticide, methidathion, degraded rapidly in a sandy loam, silt loam, clay loam and organic soil. Fifty percent of the initial applications decomposed in less than 2 weeks and more than 90% of the insecticide disappeared within 16 weeks. When soils were treated with methidathion containing(14)C in the ring or methyl side chain 40 to 66% of the radioactivity was expired as(14)CO2 after 16 weeks. Over half of the remaining radioactivity could not be extracted from the soils while lesser amounts were recovered as the parent compound or as water-soluble degradation products.In fumigated soils 50% of the initial insecticide applications still remained after 16 weeks and less than 3% of the radioactivity was expired as(14)CO2 which suggests that microorganisms are primarily responsible for the degradation of methidathion in soil.