Endosulfans I (alpha) and II (beta) dissipated in simple aqueous media in glass incubation vessels sealed with Teflon show apparent half-lives of 151 and 88 days at 30-degrees-C, respectively. These values are significantly greater than others reported in the literature, because of precautions taken to prevent volatilization and biodegradation. Endosulfan sulfate, the major oxidation product of endosulfan, is much more persistent than either of the parent isomers under the same incubation conditions. A study of the distribution of the cyclodienes endosulfans I and II, aldrin, dieldrin, and endosulfan sulfate in a sterile aqueous microbial broth showed that these compounds were distributed to the glass/medium interface. The importance of the Henry's law constant and fugacity in the nonbiological loss of these compounds and precautions that need to be taken in studies of microbial degradation are discussed.