Underlying causes of species differences in maximum life span (MLS) are unknown, although differential vulnerability of membrane phospholipids to peroxidation is implicated. Membrane composition and longevity correlate with body size; membranes of longer-living, larger mammals have less polyunsaturated fatty acid (PUFA). We determined membrane phospholipid composition of naked mole-rats (MLS > 28.3 years) and similar-sized mice (MLS = 3-4 years) by gas-liquid chromatography to assess if the approximately 9x MLS difference could be explained. Mole-rat membrane composition was unchanged with age. Both species had similar amounts of membrane total unsaturated fatty acids; however, mice had 9 times more docosahexaenoic acid (DHA). Because this n-3PUFA is most susceptible to lipid peroxidation, mole-rat membranes are substantially more resistant to oxidative stress than are mice membranes. Naked mole-rat peroxidation indices, calculated from muscle and liver mitochondrial membranes, concur with those predicted by MLS rather than by body size, suggesting that membrane phospholipid composition is an important determinant of longevity.