The bone mineral content (BMC) bone width, and cross-sectional moment of inertia (CSMI) of 141 Alaskan Eskimo tibias were measured using photon absorptiometry. The effects of age and sex on the bones' structural properties were studied. It was found that in women, BMC decreased by 50% between the third and sixth decades, but that of the males did not decline significantly with age. This was true of the CSMI as well, for bending in both the anteroposterior (AP) and mediolateral (ML) planes. This result is different from that in some other prehistoric native American populations, where tibia CSMI increases with age in both sexes. The CSMI values were significantly higher in men than in women. Also, in men the AP CSMI was 55% larger then the ML CSMI; in women this difference was only 25%, and declined with age. Since the tibia is preferentially loaded in the AP plane by locomotor activities, the platycnemic differences between the sexes may reflect sex-related differences in activity which become more pronounced with age. The non-destructive method for obtaining data on th cross-sectional geometry of dry bones which is described here may be useful in studying other archaeological collections.