Laboratory tests were performed to determine the oxidation of terbufos (Counter®),O,O-diethylS-[(tert-butylthio) methyl] phosphorodithioate, in 3 Iowa surface soils (Clarion, Harps, and Webster). The insecticide was applied to 10-g soil samples at a rate of 1 ppm, which is equivalent to a field application rate of 1 lb/acre. Soil samples were incubated at 22±1°C for 0, 1, 3, 7, 14, and 21 days. Terbufos was rapidly oxidized to its sulfoxide. The parent compound had an approximate half-life of 4 to 5 days. Terbufos sulfoxide reached a maximum concentration after 2 weeks of incubation, and terbufos sulfone did not appear until 1 week after the incubation was started. Significant differences (P=0.01) were found in the ability of the 3 soils to oxidize terbufos. The sulfoxide and sulfone had no inhibitory influence on soil microbial activities.