The genome of Dehalococcoides ethenogenes strain 195, an anaerobic dehalorespiring bacterium, contains 18 copies of putative reductive dehalogenase genes, including the well-characterized tceA gene, whose gene product functions as the key enzyme in the environmentally important dehalorespiration process. The genome of D. ethenogenes was analyzed using a bioinformatic tool based on the frequency of oligonucleotides. The results in the form of a genomic signature revealed several local disruptions of the host signature along the genome sequence. These fractures represent DNA segments of potentially foreign origin, so-called atypical regions, which may have been acquired by an ancestor through horizontal gene transfer. Most interestingly, 15 of the 18 reductive dehalogenase genes, including the tceA gene, were found to be located in these regions, strongly indicating the foreign nature of the dehalorespiration activity. The GC content and the presence of recombinase genes within some of these regions corroborate this hypothesis. A hierarchical classification of the atypical regions containing the reductive dehalogenase genes indicated that these regions were probably acquired by several gene transfer events.