The rhodoquinone (RQ) composition was studied in representatives of 20 species of Rhodospirillaceae. Thin-layer chromatography, ultraviolet spectrophotometry, mass spectrometry, and high-performance liquid chromatography revealed that six of the 20 test species contained RQ as a major quinone in average amounts of 0.28 to 2.22μmol per g dry weight of cells. The predominant homologue of RQ in the six species was as follows: RQ-8-Rhodospirillum photometricum and an unknown species resembling Rhodocyclus gelatinosus; RQ-9+RQ-10-Rhodopila globiformis; RQ-10-Rhodospirillum rubrum, Rhodopseudomonas acidophila, and Rhodomicrobium vannielii. The taxonomic significance of RQ and other isoprenoid quinones is discussed in comparison with the new classification system for the phototrophic bacteria recently proposed by IMHOFF et al.