Strain AAFu3T (= DSM 11263T = ATCC BAA-311T), a Gram-negative, non-sporulating bacterium, was isolated from a methanogenic mixed culture growing with acetone, in which acetate was the major intermediate. The cells of strain AAFu3T were slender spirilla, usually of less than one turn, and were motile by means of a single polar flagellum. The cells contained c-type cytochromes and the G+C content of the genomic DNA was 50.2 mol%. Sulfur, nitrate, fumarate, DMSO and oxygen (microaerophilically) were used as electron acceptors, but sulfate, sulfite, thiosulfate and ferric iron were not. Sulfide, hydrogen, formate and acetate acted as electron donors for respiratory growth, while fumarate, maleate and L-malate supported fermentative growth. Neither fermentative nor respiratory growth was supported by carbohydrates, fatty acids more than two carbons long, alcohols or amino acids. The strain was a mesophile. Comparative sequence analysis of the 165 rRNA gene and comparison of phenotypic characteristics showed that strain AAFu3T is closely related to Geovibrio ferrireducens, within the phylum Deferribacteres. Strain AAFu3T was designated as the type strain of a new species, for which the name Geovibrio thiophilus is proposed.