Two anaerobic bacterial strains, designated SHI-1^Tand SHI-2, were isolated from chlorinated solvent-contaminated groundwater. They were found to be identical in phenotypic properties and shared high (98.5–99.8 %) pairwise 16S rRNA gene sequence similarity. Multiple 16S rRNA genes were found to be present in the isolates as well asPelosinus fermentansDSM 17108^TandSporotalea propionicaDSM 13327^T. Strains SHI-1^Tand SHI-2 could be differentiated from their closest phylogenetic relatives,P. fermentansDSM 17108^TandS. propionicaDSM 13327^T, on the basis of their phenotypic and phylogenetic properties. The isolates were Gram-negative, spore-forming, motile rods with peritrichous flagella. Growth occurred at 10–42 °C and pH 5.5–8.5. Fermentative growth was observed on Casamino acids, fructose, fumarate, glucose, glycerol, pyruvate and yeast extract. The major organic acids produced from glucose and glycerol fermentation were propionate and acetate. The major organic acids produced from fermentation of fumarate were propionate, acetate and succinate. The major cellular fatty acids were summed feature 4 (consisting of C_{15 : 1}ω8cand/or C_{15 : 2}), summed feature 8 (consisting of C_{17 : 1}ω8cand/or C_{17 : 2}) and C_{14 : 0}dimethyl aldehyde. The polar lipids comprised aminophospholipids, including phosphatidylethanolamine and phosphatidylserine, and an unknown phospholipid. The genomic DNA G+C content was 39.2 mol%. We propose that strains SHI-1^Tand SHI-2 are assigned to a novel species of the genusPelosinus, with the namePelosinusdefluviisp. nov. (type strain SHI-1^T = NRRL Y-59407^T = LMG 25549^T). The description of the genusPelosinusis emended. We also propose the transfer ofS. propionicato the genusPelosinusasPelosinuspropionicuscomb. nov. (type strain TmPN3^T = DSM 13327^T = ATCC BAA-626^T), on the basis of phylogenetic, chemotaxonomic and phenotypic properties.