Laboratory experiments investigated the comparative degradation of six organophosphorus insecticides in soil as affected by enhanced microbial degradation. The degradation rates and product distributions of chlorpyrifos, fonofos, ethoprop, terbufos, and phorate were not dramatically altered in soils containing microbial populations adapted to rapidly degrade isofenphos. An Arthrobacter sp. isolated from soils with a history of isofenphos use rapidly metabolized isofenphos in pure culture but did not metabolize or cometabolize any of the other five organophosphorus insecticides. Likewise, only fonofos was rapidly degraded in soil with a long history of fonofos use. None of the organophosphorus insecticides were rapidly degraded in soil containing carbofuran-degrading microbial populations. Results indicate that the phenomenon of enhanced microbial degradation of soil insecticides may exhibit some degree of specificity.