The effect of organic sources on the degradation of parathion via nitrogroup reduction and hydrolysis in flooded alluvial soil was investigated. Parathion residues in the soils were extracted with solvent and separated by thin-layer chromatography. Parathion was assayed colorimetrically after its conversion to p-nitrophenol by alkaline hydrolysis. Generally, organic sources accelerated the nitrogroup reduction of parathion. The degradation followed the order glucose > rice straw > algal crust > farmyard manure > unamended. On the other hand, in soils inoculated with parathion-hydrolyzing enrichment culture, organic amendments inhibited the hydrolysis. The rate of hydrolysis in different amendments followed a reverse order of that for nitrogroup reduction. Isotope experiments revealed more rapid conversion of parathion to aminoparathion and an unidentified metabolite in rice straw amended soil than in unamended soil. These metabolites were formed rapidly also in soils amended with other organic sources. In unamended soil, a polar unidentified metabolite was detected. The common practice of incorporating organic materials in rice culture tends to influence the detoxication of parathion in the rice soil environment depending on the pathway involved.