Anaerobic microorganisms enriched from Rhine River sediments are able to remove chlorine substituents from poly‐chlorinated dibenzo‐p‐dioxines (PCDDs). A model PCDD, 1,2,3,4‐tetrachlorodibenzo‐p‐dioxin (1,2,3,4–TeCDD) was reduc‐tively dechlorinated to both 1,2,3–and l,2,4–trichlorodibenzo‐/>‐dioxins (1,2,3–and 1,2,4–TrCDD). These compounds were further dechlorinated to 1,3–and 2,3–dichlorodibenzo‐p‐dioxins and traces of 2–monochlorodibenzo‐p‐dioxin. This is the first report in the literature of the anaerobic microbial dechlorination of PCDDs. The same enrichment culture was previously found to deSchlorinate chlorinated benzenes (CBs) and polychlorinated biphenyls (PCBs). An anaerobic culture able to remove aryl chlorines from three classes of compounds has not been reported before. The rate at which the culture dechlorinates 1,2,3,4–TeCDD (t1/2 = 15.5 d) was between those observed for CBs and PCBs. This study shows that reductive dechlorination may have an effect on PCDDs in sediments, as has been demonstrated for CBs and PCBs. The formation of metabolites with a conserved 2,3‐substitution pattern from 1,2,3,4–TeCDD indicates that dechlorination of highly chlorinated dibenzo‐p‐dioxins may result in metabolites that are potentially more toxic than the parent compounds.