Low bone-density values recorded for some semiaquatic mammals indicate a morphologic compromise related to the need to perform adaptive functions both on land and in the water. However, these measurements revealed a broad range of values that did not suggest a single, most parsimonious hypothesis to explain morphologic differences in body size among taxa or the various degrees of external adaptation of these genera to an aquatic environment. With two exceptions, bone densities for all taxa were at or below average values reported in the literature for terrestrial mammals. A marked increase in deposition of compact bone was observed only in limb bones of the beaver (Castor canadensis). Analysis of variance revealed significant differences in limb-bone densities between semiaquatic and terrestrial mammals. However, density values did not differ significantly between these two groups when compared to fully aquatic mammals.