Carbofuran added to soil samples from fields previously treated with this insecticide for soil insect control broke down much faster than in soil from fields with no history of carbofuran use. In experimental field plots, a second annual application of carbofuran or fensulfothion broke down much faster than the first, and a laboratory application broke down more rapidly than the second field treatment. During the first year of field treatment, 15% granular fensulfothion applied 3 cm deep in a 12 cm wide band, 1t 60 μg AI/g (ppm) in the upper 5 cm of soil, degraded to ca. 5 ppm of toxic residues in 120 days. During the second year, an equivalent amount degraded to < 0.2 ppm in 60 days. The same concentration of carbofuran degraded to ca. 4 ppm in 60 days during the first year and to < 0.2 ppm in 30 days during the second year. Sequences of treatments of carbofuran after fensulfothion or fensulfothion after carbofuran resulted in faster breakdown of the second than of the first field treatment, but breakdown was slower than following the second application of the same compound. The increased rate of breakdown, which was assumed to be caused by selected degradative bacteria, was essentially stopped by sterilizing the soil or by lowering the pH of test soil to 4.5 or lower. Toxicity tests were conducted by bioassay using eggs and larvae of the cabbage maggot and chemical analyses were conducted on spot-check samples to confirm bioassay records.