Experiments were conducted to investigate enhanced biodegradation of carbamothioates and to evaluate the effect of microbial inhibitors on the efficacy of butylate [S-ethyl bis(2-methylpropyl)carbamothioate], EPTC (S-ethyl dipropylcarbamothioate), and vernolate (S-propyl dipropylcarbamothioate) in soils that had received butylate treatment in previous years (butylate-history soils). Inhibitors used were fonofos (O-ethyl-S-phenylether phosphonodithioate) and R-33865 (O,O-diethyl-O-phenylphosphorothioate). R-33865 and fonofos significantly improved control of large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] with butylate and EPTC in corn (Zea mays L. ‘Coker 21’). Bioassay data reflected more residual phytotoxicity from butylate and EPTC when these herbicides were combined with the microbial inhibitors. In butylate-history soils planted to soybeans [Glycine max (L.) Merr. ‘Wright’], R-33865 did not improve the efficacy of vernolate. These results indicated some cross-adaptation of the butylate-adapted microorganisms for EPTC, but no cross-adaptation was detected for vernolate. Under laboratory conditions, 14C-butylate was degraded more rapidly to 14CO2 in butylate-history soils than in non-butylate-history soils.