Four days after carbaryl-naphthyl-1-14C was mixed with soil from a field treated 6 months previously with 4 lb/A of the same insecticide, only 28% of the radiocarbon remained. Approximately 90% remained in soils with no history of pesticide applications. However, dissipation of the carbaryl-14C residues from the latter soils continued at a rather steady rate over a 120-day test period, whereas there was little dissipation after 4 days from the former. Consequently, the total 14C-residue levels were about the same, 15 to 20% of applied, when the last samples were taken. Carbaryl, per se, was the only apolar 14C-residue recovered from the soil and only small quantities, less than 2% of the amount applied, of extractable polar metabolites were encountered. Almost all of the terminal residues were unextractable from the soil with mixtures of acetone and water. Much of the loss of 14C-residues from the soil was attributed to the liberation of 14C-carbon dioxide as a result of microbial degradation of the naphthalene ring. Several fungal and bacterial isolates degraded carbaryl in the same manner as observed with soil incubations, but the rates of degradation were much slower.