Tetra- (PCE) and trichloroethene (TCE) are major groundwater pollutants due to their extensive industrial use. Several anaerobic bacteria have been isolated using chloroethenes as terminal electron acceptor. Most of these bacteria dechlorinate PCE and TCE to cis-1,2-dichloroethene (cis-1,2-DCE). A few Dehalococcoides strains are able to dechlorinate cis-1,2-DCE and vinyl chloride (VC) to the ethene. Identification of the key enzyme, the reductive dehalogenase, has revealed a new class of enzymes containing a corrinoid and two iron-sulfur clusters as cofactors. The PCE reductive dehalogenase (PceAB) of Dehalobacter restrictus and Desulfitobacterium hafniense strain TCE1 showed 100% sequence identity which raised the question of a possible horizontal gene transfer. The flanking regions of the reductive dehalogenase genes (pceAB) revealed the presence of a composite transposon (named Tn-Dha1) in strain TCE1 bordered with two identical insertion sequences ISDha1 and containing besides the already characterized pceAB, two genes (pceCT) related to members of the o-chlorophenol reductive dehalogenase gene cluster of Desulfitobacterium dehalogenans. In contrast, only the pceABCT gene cluster (i.e. without the transposon structure) was present in Dehalobacter restrictus. Various circular molecules of Tn-Dha1 indicated that Tn-Dha1 is an active mobile genetic element. The genome of Dehalococcoides ethenogenes was shown to contain eighteen copies of putative reductive dehalogenase genes. A genomic signature of D. ethenogenes was obtained by calculating the frequency of 4-letter DNA words along the genome. Local disruptions of the genomic signature were observed, corresponding to DNA, which may have been acquired by horizontal gene transfer. Fifteen putative reductive dehalogenase genes were located in such atypical regions. Moreover, several genes encoding for recombinases (transposase, integrase) were found within these atypical regions, strongly indicating that these may have been acquired horizontally.