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The influence of repeated annual field applications of the insecticide‐nematicide carbofuran on carbofuran-degrading microbial communities was studied at a site in Florida that exhibited enhanced degradation toward the chemical. Three successive annual applications of carbofuran at 4.5 kg ha 21 y 21 did not result in an increase in the size of the microbial community capable of mineralizing uniformly ringlabeled 14 C-carbofuran (carbofuran-ring degraders) in surface soil (0‐15 cm depth). After the second annual application, however, the community capable of mineralizing carbonyl-labeled 14 C-carbofuran (carbofuran hydrolyzers) in the treated surface soil was significantly larger than that after the first annual application. Communities of methylamine utilizers in treated and untreated soils were much larger than the communities of carbofuran phenol degraders, but not statistically different from the sizes of carbofuran hydrolyzers. In addition to no increase in the number of carbofuran ring degraders in the treated site during three consecutive annual applications of carbofuran, laboratory addition of 10 mg carbofuran g 21 to treated and untreated soils collected in 1995 did not result in an increase in the number of carbofuran ring degraders. This suggests that degradation of the ring structure of carbofuran in soil is a cometabolic process. q 2001 Elsevier Science Ltd. All rights reserved.