Mammalian sperm have unusual plasma membranes compared to those of somatic cells. After leaving the testes, sperm cease plasma membrane lipid and protein systhesis. A major fraction of mammalian sperm plasma membranes are lipid linked. A large fraction of their lipid chains are highly unsaturated. Biophysical studies reveal that lipids are regionalized on the sperm surface and are highly immobile. This immobile fraction evolves with sperm development. This non-diffusing fraction is also observed in bilayers reconstituted from lipid extracts of sperm head plasma membranes, suggesting the existence of gel phase domains in these membranes. This hypothesis is further supported by differential scanning calorimetry, which shares at least two relatively broad phase transitions with physiological temperature falling between these major transitions.