The major classes of neutral lipids and the molecular species of choline, ethanolamine, serine, and inositol phosphatides were determined in the sarcotubular membranes of the skeletal muscle of rabbit, rat, chicken, and man. All membrane preparations contained qualitatively the same lipid classes, but certain significant quantitative differences occurred. The largest discrepancies were noted in the phosphatidylethanolamines, which ranged from about 17% in rabbit to 24% in man and also varied in their plasmalogen content. The sarcotubular membranes of the rat, rabbit, and man contained minor amounts of cardiolipin (2%), which may have represented a mitochondrial contamination. Despite considerable variation in the component fatty chains, the molecular weight distributions of corresponding glycerolipids showed considerable similarity among the different animal preparations. The phosphatidylethanolamines of the rat, however, contained large amounts of 16:0 22:6 and 18:0 22:6 at the expense of 16:0 18:1 and 18:0 18:1, whereas in the phosphatidylserines of the rat and chicken, the 18:0 22:6 was substituted for 18:0 22:4 in the rabbit and man. The membranes contained variable amounts of neutral lipid, and gave molar ratios of phospholipid to cholesterol ranging from 2 to 4. The triglycerides possessed similar structures in all four membrane preparations. Comparisons with the molecular species of lipids of other tissues revealed significant differences in the constitution of all glycerolipids except phosphatidylinositols and cardiolipins, which were similar in all membranes. Little variation was also seen in the molecular species of the sphingomyelins.