A novel obligately anaerobic, non-spore-forming, rod-shaped mesophilic, halophilic, Gram-stain-negative bacterium, was isolated from sediments of Guaymas Basin. The strain, designated Ra1766G1(T), grew at 20-40 degrees C (optimum, 30-35 degrees C) and at pH 6.0-8.0 (optimum, pH 6.5-7.5). It required 0.5-7.5 % NaCl (optimum, 2-3 %) for growth. Sulfate, thiosulfate, elemental sulfur, sulfite, fumarate, nitrate and nitrite were not used as terminal electron acceptors. Strain Ra1766G1(T) used cellobiose, glucose, mannose, maltose, arabinose, raffinose, galactose, ribose, sucrose, pyruvate and xylose as electron donors. The main fermentation product from glucose metabolism was acetate. The predominant cellular fatty acids were anteiso-C-15:0, iso-C-15:0, anteiso DMA-C-15:0 and C-16:0. The main polar lipids consisted of diphosphatidylglycerol, phosphatidylglycerol, iso-DMA-C-15:0 glycolipids and phospholipids. The G+C content of the genomic DNA was 31.2 mol%. The closest phylogenetic relatives of strain Ra1766G1(T) were Natranaerovirga pectinivora AP3(T) (92.4 % 16S rRNA gene sequence similarity), Natranaerovirga hydrolytica APP2(T)(90.2 %) and Defluviitalea saccharophila 6LT2(T) (88.9 %). On the basis of phylogenetic inference and phenotypic properties, strain Ra1766G1(T) represents a novel species of a new genus for which the name Vallitalea guaymasensis is proposed. The type strain of the type species is Ra1766G1(T) (=DSM 24848(T)=JCM17997(T)).