Indigo-reducing, obligately alkaliphilic strains A11T, F11 and F12 were isolated from indigo fermentation liquor obtained from Tokushima Prefecture, Shikoku, Japan. The isolates grew at pH 9.0-12.3, but not at pH 7.0-8.0. The optimum pH range for growth was 9.5-11.5. They were Gram-negative, facultatively anaerobic, rod-shaped strains with peritrichous flagella. The isolates grew in 0-14 % (w/v) NaCl, with optimum growth at 1-11 %. They grew at temperatures of 15-35 degrees C with optimum growth at around 20-30 degrees C. dl-Lactate was the major end product from d-glucose. No quinones were detected. The peptidoglycan type was A4 alpha, l-Lys (l-Orn)-d-Asp. The major cellular fatty acids were C16 : 0, C16 : 17c and C18 : 19c. The DNA G+C contents were 47.0-47.8 mol%. Phylogenetic analysis based on 16S rRNA gene sequence data indicated that the isolates belong to the genus Alkalibacterium. DNA-DNA hybridization revealed low relatedness values between the isolates and the three phylogenetically most closely related species, Alkalibacterium olivapovliticus, Alkalibacterium psychrotolerans and Alkalibacterium iburiense (<41 %). On the basis of phenotypic characteristics, including hydrolysis of cellulose and fermentation of carbohydrates, and chemotaxonomic characteristics, phylogenetic data and DNA-DNA relatedness data, it is concluded that the isolates merit classification as representatives of a novel species of the genus Alkalibacterium, for which the name Alkalibacterium indicireducens sp. nov. is proposed. The type strain of this species is A11T (=JCM 14232T=NCIMB 14253T).