1. The phospholipids from the non-capsulated strain of Pneumococcus I-192R, A.T.C.C. 12213, were separated into three fractions by chromatography on columns of silicic acid and DEAE-cellulose (acetate form). 2. The water-soluble phosphate esters produced by deacylation of each fraction were separated by chromatography on columns of DEAE-cellulose (HCO(3) (-) form). 3. Three deacylated products, diglycerol phosphate, glycerylphosphorylglycerol phosphate and bis(glycerylphosphoryl)glycerol, were identified by analysis, by chemical degradations and by comparison with synthetic materials. 4. From a study of freshly isolated lipids prepared and worked up under conditions where exposure to acid was minimal, it was concluded that the Pneumococcus contains phosphatidylglycerol and bisphosphatidylglycerol, in the molar proportion 1:2.5-3.0, and that the deacylation product glycerylphosphorylglycerol phosphate was probably an artifact of the isolation procedure. 5. Acid-catalysed isomerization (phosphodiester migration) of diglycerol phosphate and bis(glycerylphosphoryl)glycerol and transesterification (glycerol phosphate transfer) of diglycerol phosphate were observed. The structures of the products were established by degradation. 6. A novel mechanism for the biosynthesis of bisphosphatidylglycerol is presented.