THE lipids of biological membranes are generally highly heterogeneous in both head group and fatty acid composition. The membrane lipids of most organisms contain an assortment of fatty acids of various hydrocarbon chain lengths and configurations; the cytoplasmic membrane lipids of Escherichia coli, for example, contain saturated, cis-monounsaturated and cis-cyclopropane fatty acids of 14–19 carbons1. To date, the role(s) of this fatty acid heterogeneity remains largely unclear. We report here that by using an antilipogenic compound and by adding a single fatty acid to the culture medium of the simple cell wall-less procaryote Acholeplasma laidlawii B, it is possible essentially to abolish lipid fatty acid heterogeneity without any obvious effects on cell growth. Furthermore, the membrane lipids of cells cultured in these conditions show a greatly sharpened thermotropic gel-to-liquid crystalline phase transition, indicating that neither fatty acid heterogeneity nor a low-cooperativity lipid-phase transition are essential for proper membrane function in this organism.