Monoglucosyl diglyceride and diglucosyl diglyceride are the dominant lipids of the Acholeplasma laidlawii membrane. Diglucosyl diglyceride forms a lamellar liquid crystalline phase with water while monoglucosyl diglyceride forms a reversed hexagonal phase. Depending on the amounts of unsaturated acyl chains of the lipids, a mixture of monoglucosyl diglyceride and diglucosyl diglyceride forms lamellar or reversed cubic phases at physiological temperatures. A high degree of cis unsaturation favors formation of the cubic phase with increasing monoglucosyl diglyceride content. The structure of the cubic phase is composed of aggregates, where the lipids can diffuse over macroscopical distances. A structure containing close-packed spherical micelles is therefore ruled out, and the NMR diffusion data are compatible with other previously proposed cubic bicontinuous structures [Luzzati, V., & Spegt, P. A. (1967) Nature (London) 215, 701; Scriven, L. E. (1976) Nature (London) 263, 123; Lindblom, G., Larsson, K., Johansson, L. B.-A., Fontell, K., & Forsén, S. (1979) J. Am. Chem. Soc. 101, 5465]. Monoglucosyl diglyceride/diglucosyl diglyceride ratios forming cubic phases have not been observed in vivo. It is concluded that formation of the cubic phase is strongly dependent on the molecular shape of the lipids. The results are significant for the physiological regulation of the lipid composition in A. laidlawii membranes as well as for the function and organization of biological membranes in general.