1. The specimen of sea-urchin semen examined contained 0.95% lipid material consisting of 13.6% neutral fat, 32.9% free fatty acids, 26.0% phospholipids, 9.2% sterols and 18.3% other unsaponifiable material. 2. The component fatty acids of the non-phosphorus-containing lipids have been computed to be: palmitic 10.1, unsaturated C16 2.1 (-3.0), C18 30.4 (-4.9), C20 45.1 (-7.0), C22 12.3 (-6.5)%, w/w, this being a composition which can be regarded as typical of a marine animal fat. 3. The N : P ratio of the phospholipids indicated the presence of mono- and diaminophosphatides and possibly the presence of non-phosphorus-containing lipoproteins. 4. Adequate reserves of non-phosphorus-containing lipids are present in the spermatozoa, in addition to phospholipids to serve as a source of energy required for movement.