There are only minor changes in the amounts of the major lipid constituents of Dictyostelium discoideum plasma membranes during the early stages in the differentiation of this organism. By the time cells reach the pseudoplasmodial stage of development there are small increases in the amounts of phosphatidylinositol, phosphatidylglycerol, and lysophosphatidylethanolamine, and small decreases in the amounts of phosphatidylethanolamine and its plasmalogen form. There is also a slight decrease in the total amount of sterol in the plasma membrane during the transition from aggregation to pseudoplasmodium formation. However, no significant change in membrane fluidity as determined by electron paramagnetic resonance (EPR) accompanies these minor changes in lipid composition. It can be concluded that the establishment of cell-cell interaction in D. discoideum does not involve gross changes in plasma membrane fluidity or lipid composition. It was found that the plasmalogen form of phosphatidylethanolamine is a major phospholipid constituent in D. discoideum, and that this species is somewhat enriched in the plasma membrane.-Weeks, G., and F. G. Herring. The lipid composition and membrane fluidity of Dictyostelium discoideum plasma membranes at various stages during differentiation.