Dioxin-dehalogenating cultures of anaerobic bacteria were established by successive transfers of laboratory microcosms that were initially prepared with contaminated sediment from the creek Spittelwasser site, Germany. The samples have been obtained from different depths of the sediment and were highly polluted with polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) and other organohalogen compounds. The enrichment cultures were able to reductively dehalogenate the spiked model compounds 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD), 1,2,3-trichlorodibenzo-p-dioxin (1,2,3-TrCDD), and 1,2,4-trichlorodibenzo-p-dioxin (1,2,4-TrCDD). Different dechlorination pathways were elucidated that exhibited different dechlorination endpoints and variable ratios of the major products 1,3- and 2,3-dichlorodibenzo-p-dioxin (DiCDD). Reductive dechlorination of spiked model compounds occurred also with sediment material from low contaminated sites. The results of a ribosomal DNA-based molecular survey (nested PCR) confirmed the presence of potential dechlorinating populations including Desulfitobacterium and Dehalococcoides, which were present in all cultures. The amplified Dehalococcoides 16S rDNA fragment was identical to the sequence of strain CBDB1. This bacterium was tested for its ability to dechlorinate dioxins and could transform 1,2,3,4-TeCDD, 1,2,3-TrCDD, 1,2,4-TrCDD, and 2,3-DiCDD to dechlorination products with 2-monochlorodibenzo-p-dioxin as tentative end product. In addition, the environmentally relevant congener 1,2,3,7,8-pentachlorodibenzo-p-dioxin was transformed to 2,3,7,8-tetrachlorodibenzo-p-dioxin and other lesser chlorinated congeners including 2,7- or 2,8-dichlorodibenzo-p-dioxin. Another study with the enrichment cultures showed that 1,2,3-trichlorobenzene, which was supplied using a two-phase system with hexadecane, was dechlorinated to 1,3-dichlorobenzene. After chlorobenzene dechlorination, the number of dioxin dechlorination was increased by three orders of magnitude. Molecular biological techniques have been applied in order to identify the key microbes. Restriction fragment length polymorphism (RFLP) analysis of 16S rDNA libraries revealed the enrichment of ten different restriction patterns including a Dehalococcoides-like organism and a bacterium of the Cytophaga-Flavobacterium-Bacteroides group. Another method, single-strand conformation polymorphism analysis, showed the additional enrichment of a bacterium similar to Trichlorobacter thiogenes.