Measurements of volume and weight for limb bones from a series of Recent mammals resulted in the recognition of two groups corresponding to different habitats, aquatic and terrestrial. Most aquatic mammals studied exhibited significantly higher limb-bone density than did the terrestrial mammals. It is proposed that the higher bone density of aquatic mammals is an adaptation to reduce problems of buoyancy. Cetaceans and some "pinnipeds" hav e secondarily reduced bone density with the acquisition of lung collapse during deep dives. The greater bone density of aquatic mammals is at least partially achieved through increased deposition of compact bone along the shafts of limb bones. As a result, it is possible to differentiate visually aquatic from terrestrial limb elements by sectioning the bone and examining the percentage of compact bone present. Fossil bones can be interpreted in the same manner, thus providing a quantifiable test for the determination of life habits in extinct mammals.--Journal abstract.