This report sought to determine the initial oxidative pathways for dibenzodioxin and dibenzofuran compounds by bacteria in aquatic systems. Bacterial cultures were isolated and oxidized dibenzo-p-dioxin and mono-chlorinated dibenzo-p-dioxins; however, di-chlorinated dibenzo-p-dioxins, were not metabolized by either strain. The metabolite, 1,2-dihydrodibenzo-p-dioxin, was a potent inhibitor of further oxidation. Pseudomonas sp. N.C.I.B. 9816, strain II, when grown on salicylate in the presence of dibenzo-p-dioxin, accumulated cis-1,2-dihydroxy-1,2-dihydrodibenzo-p-dioxin and 2-hydroxydibenzo-p-dioxin in the culture medium. Crude cell extracts prepared from the parental strain grown with naphthalene oxidized cis-1,2-dihydroxy-1,2-dihyrodibenzo-p-dioxin under both aerobic and anerobic conditions to 1,2-dihydroxydibenzo-p-dioxin. Further degradation of this metabolite was not detected. Whole cells of the parent strain of Beijerinckia, grown with succinate and biphenyl, oxidized dibenzo-p-dioxin and several chlorinated dioxins. The rate of oxidation of the chlorinated dibenzo-p-dioxins decreased with an increasing degree of chlorine substitution. A mutant strain (B8/36) of Beijerinckia oxidized dibenzo-p-dioxin to cis-1,2-dihydroxy-1,2-dihydrodibenzo-p-dioxin.