An evaluation of the fracture characteristics of bovine tibia compact tension specimens associated with controlled crack propagation in the longitudinal direction has been made. The fracture mechanics parameters of critical strain energy release rate (Gc) and critical stress intensity factor (K_c) were determined for a range of crack velocities. A comparative fracture energy (W) was also evaluated from the area under the load-deflection curve. It was found that an increase in the average crack velocity from 1.75 to 23.6×10⁻⁵ m sec⁻¹ produced increases in Gc (from 1736 to 2796 J m−2), K_c (from 4.46 to 5.38 MN m^−3/2) and W. At crack velocities >23.6×10⁻⁵ m sec⁻¹, W decreased appreciably. Microstructural observations indicated that, for crack velocities <23.6 m sec⁻¹, relatively rough fracture surfaces were produced by the passage of the crack around intersecting osteons (or lamellae), together with some osteon pull-out. In contrast, at a higher crack velocity, fracture was characterized by relatively smooth surfaces, as the crack moved indiscriminately through the microstructural constituents.