The degradation of alpha and beta isomers of endosulfan by a two-member bacterial co-culture was studied. Results were similar whether the two isomers were present individually or together, as in technical endosulfan. The degradation of both isomers was accompanied by the formation of endosulfan diol and endosulfan lactone. Accumulation of the metabolite, endosulfan sulfate was, however, not observed during the reaction with either of the isomers. The microbial degradation of endosulfan isomers was also accompanied by a decrease in its toxicity to the test organism Tubifex tubifex Muller.