Rhodopseudomonas cryptolactis, sp. nov., was enriched from a hot spring environment using conditions that favor growth of anoxygenic phototrophs able to use N2 as the sole nitrogen source for growth. Phototrophic growth rate of the budding bacterium is optimal at ca. 40°C, and the maximum growth temperature is ca. 46°C. R. cryptolactis requires vitamin B12, nicotinic acid, and p-aminobenzoic acid for optimal growth in synthetic media, and has restricted capacities in respect to phototrophic carbon and nitrogen nutrition. Molecular nitrogen, NH4+, and urea can serve as nitrogen sources; pyruvate and lactate as carbon sources. Utilization of lactate, however, is dependent on the presence of bicarbonate, suggesting a linkage between lactate photometabolism and operation of the Calvin reductive pentose cycle. The photopigments of R. cryptolactis, including bacteriochlorophyll a, are housed in lamellar stacks parallel to and underlying the cell membrane. When grown at low light intensity, the cells apparently produce an additional species of light- harvesting Bchl that has a major in vivo absorbancy peak at 822 nm. The bacterium also grows as an aerobic chemoheterotrophs in darkness, and the presence of O2 causes severe repression of photopigment synthesis.