Kendaia clay loam contained more than 105 microbial cells per g able to convert 14C-carbonyl-labelled carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate) to ~4CO2 but never more than 130 cells per g transforming 14C-ring-labelled carbofuran to CO 2. The sizes of the population rarely increased as a result of addition of the insecticide to soil. Mineralization of these compounds proceeded with little or no acclimation phase, and subsequent additions were usually metabolized more readily, except at 10 mg of carbofuran per kg or if subsequent additions of the pesticide were made long after the first. More than 60% of the UC in the carbonyl but less of the ~4C in the ring was microbiologically converted to ~4CO 2 in this soil. Streptomycin and cycloheximide each inhibited conversion of the carbonyi or ring carbon to CO 2. Urea but not NH4NO 3 markedly inhibited the conversion of the carbonylqabelled insecticide to ~4CO2. The addition of glucose and succinate together with the insecticide did not enhance mineralization of ring-or carbonyl-labelled carbofuran. The data suggest that soils containing a large population of microorganisms able to convert the carbonyl carbon to CO~ will not show a marked effect of prior treatment with the insecticide and that few organisms individually are able to mineralize the ring.