The degradation of epichlorohydrin (3-chloropropylene oxide or 1-chloro-2,3-epoxypropane) by whole-cell suspensions of Xanthobacter strain Py2 was investigated. Cell suspensions prepared from cultures grown with propylene as the carbon source readily degraded epichlorohydrin. The ability to degrade epichlorohydrin correlated with the expression of enzymes involved in alkene and epoxide metabolism, since cell suspensions prepared from cultures grown with glucose or acetone, in which the enzymes of alkene and epoxide oxidation are not expressed, did not degrade epichlorohydrin. The alkene monooxygenase-specific inhibitor propyne had no effect on the degradation of epichlorohydrin, demonstrating that alkene monooxygenase is not involved in epichlorohydrin conversion. The interaction of epichlorohydrin and epibromohydrin with the epoxidase which catalyzes aliphatic epoxide conversions was established by showing that the epihalohydrins were specific and potent inhibitors of propylene oxide-dependent O 2 consumption by cell suspensions. The rates of degradation of epoxides in whole-cell suspensions decreased in the series propylene oxide > epifluorohydrin > epichlorohydrin > epibromohydrin. The pathway of epichlorohydrin degradation was investigated and found to proceed with stoichiometric dechlorination of epichlorohydrin. The first detectable product of epichlorohydrin degradation was chloroacetone. Chloroacetone was further degraded by the cell suspensions, and in the process, acetone was formed as a nonstoichiometric product. Acetone was further degraded by the cell suspensions with enzymes apparently induced by the accumulation of acetone. The metabolism of allyl chloride (3-chloropropylene) by propylene-grown cells was initiated by alkene monooxygenase and proceeded through epichlorohydrin, chloroacetone, and acetone as intermediate degradation products. These studies reveal a new pathway for halogenated epoxide degradation which involves halogenated and aliphatic ketones as well as other unidentified intermediates and which is unique from previously characterized hydrolytic degradative pathways.