The composition of Rhodopseudomonas spheroides grown under oxygen or under air in the dark, or semi-anaerobically in the light was studied. The amounts of various constituents were expressed per cell. Anaerobically grown cells were 20% lighter than cells grown under oxygen but contained approximately 50% more protein and 85% more phospholipid. The differences in weight were largely due to greater amounts of reserve material (poly-$\beta $-hydroxybutyrate and carbohydrate) in oxygen-grown cells. The major increase in the protein was due to a doubling of the 'particulate' protein, the 'soluble' protein increasing by only 20%. The values of certain constituents in cells grown in air were intermediate between those of oxygen-grown cells and semi-anaerobically grown cells. The changes in composition were followed during adaptation from growth under oxygen to semi-anaerobic conditions in the light. Particulate protein, phospholipid, and enzymes concerned in bacteriochlorophyll synthesis increased markedly before photopigments or chromatophores were formed. These results indicate the sequence of some of the steps concerned in the differentiation of the cytoplasmic membrane of pigment-free micro-organisms into the fully formed photosynthetic apparatus.