Two new acidophilic bacteria (strains M-1 and M-2) were isolated from coal mine waters, characterized, and placed in the genus Pseudomonas, as Pseudomonas acidophila, sp. n. Obligate acidophiles unable to grow at neutrality, they are capable of rapid and vigorous growth at pH 3.5, with a range of pH 2.5-4.5, in an especially devised yeast extract-glucose salts medium. Unlike the ferrobacilli and thiobacilli, they are heterotrophs rather than autotrophs. They differ from other acid-tolerant heterotrophic bacteria of such environments in being obligate acidophiles and fail to grow on routine bacteriological culture media. Various morphological and physiological characteristics of the two strains were determined. A low level of yeast extract (0.025-0.050%) stimulates growth, but growth is progressively inhibited above 0.1%. Both inorganic and organic nitrogen sources are utilized for growth. Growth with ammonium salts lowered the pH of the medium but growth with organic nitrogen sources such as L-histindine, hydrolyzed casein or urea consistently raised the pH from an initial value of 4.0 to 5.0 or higher.