Eight strains of Gram-negative bacteria able to form ring-like cells were isolated from Mahoney Lake, a meromictic lake in south-central British Columbia, Canada. All strains were pink-purple and contained bacteriochlorophyll a incorporated into the light-harvesting 1 and 2 and reaction-centre pigment-protein complexes. Growth did not occur anaerobically under illuminated conditions; these strains were obligately aerobic, prompting their designation as members of the aerobic phototrophic bacteria. Physiological characterization revealed that these isolates share a similar tolerance to high levels of salinity and pH, as would be expected of bacteria from a highly saline lake; however, the strains exhibited marked differences in their ability to utilize organic substrates for aerobic heterotrophic growth. 16S rRNA sequence analysis showed that the strains are closely related to members of the non-phototrophic genera Octadecabacter (92.0-92.9%) and Ketogulonicigenium (92.2-92.6%), as well as to aerobic phototrophs of the genera Roseivivax (92.2-92.9%) and Roseovarius (91.7-92.4%) within the 'Alphaproteobacteria'. The DNA G+C content was 66.2 mol%. The unusual light-harvesting complex 2, the distinct morphological features and physiological traits of these strains as well as the phylogenetic data support the proposal of the novel genus and species Roseicyclus mahoneyensis gen. nov., sp. nov., with ML6(T) (=DSM 16097(T)=VKM B-2346(T)) as the type strain.