Bone mineralization and bone geometry at femoral midshaft were analysed for 33 muskoxen Ovibos moschatus ranging in age from 3 weeks to 4.5 years and in body mass from 9 to 215 kg, to assess changing mechanical properties associated with postnatal ontogeny. Analysis of the mineral density of bone (BMD) indicates little or no change in femoral mineralization until after 4 months of age. Between 4 and 6 months of age BMD increases markedly, and complete mineralization occurs by about 18 months of age. Femoral length (L) and bone strength (as measured by Z, the section modulus) scale negatively allometric with body mass (L ∞ M0.26 and Z ∞ M0.88) indicating that neonate muskoxen have longer and stronger femora for their body size than do older animals. When the neonate data were excluded, however, bone length and bone strength regressed against body mass did not differ significantly from isometry (L ∞ M0.32 and Z ∞ M1.03), implying that different growth trajectories characterize postnatal ontogeny before and after 4 months of age. As a consequence of the timing of ontogenetic changes in bone mineralization and cross-sectional geometry, safety factors calculated for the femur (derived from estimates of peak bending stress and fracture strength) are largest and smallest in neonates and 4-month-old calves, respectively. It is hypothesized that increased bone mineralization and accompanying diaphyseal stiffness after 4 months of age are necessary to maintain structural integrity of the femur.