The phylogenetic position of an oxygenic photosynthetic prokaryote containing chl d as a major pigment, which have been tentatively named “Acaryochloris marina,” was analyzed using small subunit rDNA sequences. Phylogenetic relationships inferred among A. marina, selected strains from the Cyanobacteria, and plastids showed that A. marina was within the cyanobacterial radiation. The A. marina lineage diverged independently from other phylogenetic subgroups of the Cyanobacteria. No organism was found to be identical or related closely to A. marina by a similarity search and phylogenetic analysis. Based on these results, in addition to the reported characteristics of the cell morphology, pigment composition, and photosynthesis, a new taxon, Acaryochloris marina Miyashita et Chihara gen. et sp. nov., is formally proposed for the oxy-genic photosynthetic prokaryote.