Two strictly anaerobic bacterial strains, designated IP3-3^T and SBP-1, were isolated from groundwater contaminated by chlorinated alkanes and alkenes at a Superfund Site located near Baton Rouge, Louisiana (USA). Both strains reductively dehalogenate a variety of polychlorinated aliphatic alkanes, including 1,2-dichloroethane, 1,2-dichloropropane, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane and 1,2,3-trichloropropane, when provided with hydrogen as the electron donor. To clarify their taxonomic position, strains IP3-3^T and SBP-1 were characterized using a polyphasic approach. Both IP3-3^T and SBP-1 are mesophilic, non-spore-forming, non-motile and Gram-stain-negative. Cells of both strains are irregular cocci with diameters of 0.4–1.1 µm. Both are resistant to ampicillin and vancomycin. The genomic DNA G+C contents of strains IP3-3^T and SBP-1 are 55.5±0.4 and 56.2±0.2 mol% (HPLC), respectively. Major cellular fatty acids include C_{18 : 1}ω9c, C_{16 : 0}, C_{14 : 0} and C_{16 : 1}ω9c. 16S rRNA gene sequence based phylogenetic analyses indicated that the strains cluster within the phylum Chloroflexi most closely related to but distinct from the species Dehalogenimonas lykanthroporepellens (96.2 % pairwise similarity) and Dehalococcoides mccartyi (90.6 % pairwise similarity). Physiological and chemotaxonomic traits as well as phylogenetic analysis support the conclusion that these strains represent a novel species within the genus Dehalogenimonas for which the name Dehalogenimonas alkenigignens sp. nov. is proposed. The type strain is IP3-3^T ( = JCM 17062^T = NRRL B-59545^T).