Phospholipid compositions were radiochemically studied on spherical bacteria belonging to the genera of Micrococcus, Planococcus, Staphylococcus, and Sporosarcina, and two unidentified motile cocci. The strains of the genera of Planococcus and Sporosarcina exhibited the presence of phosphatidylethanolamine (PE), cardiolipin (CL), phosphatidylglycerol (PG), and an unidentified phospholipid b. The genus Micrococcus showed the presence of CL, PG, and unidentified phospholipids a and b, and the genus Staphylococcus, the presence of CL, PG, and the unidentified phospholipid b. The phospholipid composition indicated a close relation to other taxonomic characteristics in this group of bacteria, and was not affected qualitatively by cultural conditions. From these findings, the phospholipid composition is considered to be useful for differentiation of aerobic gram-positive cocci.Since motile cocci were first isolated from water in 1889 by ALI-CoHEN (1), the isolation of this kind of bacteria has been reported by other investigators (2, 3). In 1894, MIGULA (4) proposed the genus Planococcus for them on the basis of their motility. This genus was accepted by a few taxonomists, such as CHESTER (5), VUILLEMIN (6), and KRASSILNIKOV (7). The majority of other taxonomists have included these bacteria in the genus Micrococcus (8-11), because motile cocci could not be differentiated from the members of the genus Micrococcus except for their motility.Aerobic, gram-positive, non-motile cocci were divided into two groups on the basis of DNA base composition (GC-content in DNA) (12)(13)(14). Strains of the genus Staphylococcus showed a low GC-content in DNA of 30.8 % to 36.5 %, and those of Micrococcus from 69 % to 75 % (12), while motile cocci showed the GC-content in DNA of 40% to 50% (15-17). Therefore, motile cocci differed 97