Sarcoplasmic reticulum (SR) membranes isolated from rabbit and lobster muscles have similar phospholipid classes, but they differ in plasmalogen content. The plasmalogenic species are mostly distributed among phosphatidylethanolamines (PE's) and make up about 62% of the total in rabbit SR and about 46% in lobster membranes. Lobster SR phospholipids contain large amounts of polyunsaturated fatty acids which are present in low amounts in rabbit membranes. The total unsaturated fatty acids of phosphatidylcholines (PC's) represent about 53% and 73% of the total fatty chains for rabbit and lobster SR, respectively. The values found for PE's were about 56% and 64%, respectively. Furthermore, lobster membranes contain significant amounts of PC and PE molecular species with unsaturated fatty acids in positions 1 and 2, whereas rabbit SR contain low amounts.