A gram-positive, motile, rod-shaped, strictly anaerobic, sporulating bacterium was isolated from an enrichment initiated with mullet gut contents. The organism grew optimally at 30 degrees C and pH 6.5, and at a salinity of 1/10(3). Out of a variety of polysaccharides tested as growth substrates, only alginate supported growth in either semidefined or complex culture medium. The organism also grew on a variety of mono- and disaccharides. Moles product per 100 mol of alginate monomer degraded were: acetate, 186; ethanol, 19; formate, 54; and CO2, 0.19. Moles product per 100 mol of hexose in cellobiose or glucose degraded were: acetate, 135; ethanol, 61; formate, 63; and CO2, 61. Hydrogen was not detectable during the incubations (detection limit, < 10(-5) atm) and propionate, butyrate, lactate, or succinate were not produced as fermentation end products (< 2 mol per 100 mol of monomer). The G+C content of DNA from the bacterium was 30.2 +/- 0.3 mol%, and the cell walls contained the peptidoglycan component meso-diaminopimelic acid. A phylogenetic analysis of the 16S rDNA sequence indicated that the organism grouped closely with members of the RNA-DNA homology group 1 of the genus Clostridium. However, it differed from other species of the genus with regard to morphology, growth temperature optimum, substrate range, and fermentation pattern and is therefore designated as a new species of Clostridium; the type strain is A-1 (DSM 8605).