Acidiphilium aminolytica is proposed for a species of the genus Acidiphilium. Acidiphilium aminolytica can be phenotypically differentiated from all other species of the genus Acidiphilium. The seven strains of this species that have been studied are Gram-negative, aerobic, mesophilic, non-sporeforming, motile, and rod-shaped bacteria. They grow between pH 3.0 and 6.0, but not at pH 6.5. They yield positive results in tests for hippuric acid hydrolysis, catalase and urease production. Oxidase, esculin hydrolysis, and beta-galactosidase tests are negative. They can use D-glucose, D-galactose, inositol, sorbitol, L-lysine, L-glutamate, L-arginine, beta-alanine, DL-4-aminobutyrate, DL-5-aminovalerate, sperimine, or diaminobutane as a sole carbon source, but cannot use elemental sulfur and ferrous iron as an energy source. The DNA base composition is 58.7-59.2 G + C mol%. The major isoprenoid quinone is ubiquinone with ten isoprene units (Q-10). The major fatty acid is the C18:1 fatty acid. Two ornithine amide lipids, the C18:1 fatty acid esters of alpha-N-3-hydroxystearylornithyltaurine and alpha-N-3-hydroxystearylornithine, are detected as the polar aminolipid. DNA relatedness between this species and the other species of Acidiphilium, the genera Acidomonas, and Acidobacterium was 29 to 2%. These results indicate that this new species should be placed in the genus Acidiphilium. The type strain (strain 101) of A. aminolytica is JCM 8796.