The fatty acid composition of 9 different tissues and organs of the female horseshoe crab Xiphosura (Limulus) polyphemus — one of the very few recent representatives of the ancient arthropod class Merostomata — was investigated in reference to the distribution of fatty acids through the marine food web. Fatty acid spectra, in which polyunsaturated fatty acids are predominant, especially eicosapentaenoic acid (20:5 ω3), revealed features characteristic of marine lipids. However, rather large quantities of monoenoic fatty acids also occur in all organs. In the saturated fatty acid fractions, the high content of branched-chain components is worth noticing, particularly in the gills and the carapace (35%); in all probability, the high amount of the branched-chain fatty acids is associated with their protective function in surface lipids. Isoprenoid fatty acids such as pristanic and phytanic acid were absent.