Bull, boar, rabbit and human semen was fractionated into seminal plasma, sperm tails and sperm heads. The fatty acid composition of total lipid, phopholipid, phosphatidyl choline, triglycerides and diglycerides was determined in each fraction. Qualitative and quantitative differences in the fatty acid contents were found between the species and between the semen fractions. Bull and rabbit seminal plasma contained relatively higher proportions of polyunsaturated acids (PUFA) (ω6 and ω3) than that of man and boar. The sperm tails contained in all four species more PUFA than did the sperm heads. The higher metabolites of oleic acid were not present in measurable amounts. The 20- and 22-carbon acids were present in semen in much larger quantities than thus far reported, in any other mammalian tissue.