Four Gram-negative strains (ML4(T), ML19, ML31, ML32) of nonmotile, appendaged, budding bacteria were isolated from the meromictic Mahoney Lake in British Columbia, Canada. The strains were red to brown-red in color and produced bacteriochlorophyll a incorporated into photosynthetic pigment-protein complexes. Phylogenetic analysis has placed these strains within the class Alphaproteobacteria, with the closest relatives being members of the genera Erythrobacter, Porphyrobacter, and Erythromicrobium. Morphological features warrant their inclusion within the genus Porphyrobacter and these strains can be readily distinguished from other species of this genus on the basis of a mesophilic temperature range, a broad pH range, and tolerance to extremely high NaCl and Na2SO4 concentrations, in keeping with the environment from which they were isolated, a Na2SO4-dominated meromictic lake. These isolates utilize a variety of organic substrates for aerobic chemoheterotrophic growth and do not grow under anaerobic conditions, in either the presence or the absence of light. All strains require vitamin B-12, and strains ML4(T) and ML19 require biotin. The DNA G + C contents ranged from 62.2 to 64.9 mol%. Phenotypic and phyletic data support the classification of strains ML4(T), ML19, ML31, and ML32 as a novel Porphyrobacter species for which the name Porphyrobacter meromictius sp. nov. is proposed.