Enrichments of pH: 5.1, inoculated from a warm, acidic sulfur spring of the Yellowstone Park, yielded purple red cultures of a mobile, spherical organism belonging to the Rhodospirillaceae, described herein as a new species Rhodopseudomonas globiformis. Under favourable conditions, the cells are 1.6–1.8 μm in diameter, smaller and larger cells may occur. The photopigments consist of bacterio-chlorophyll aP and new aliphatic methoxylated ketocarotenoids. The organism grows either under anaerobic conditions in the light or under microaerophilic conditions in the dark. Biotin, p-aminobenzoic acid and a source of reduced sulfur are required as growth factors. Gluconate, mannitol, fructose and ethanol are the best carbon sources at a pH of 4.9. Growth is inhibited by low concentrations of sulfide.