Dynamic compression results are reported for single-crystal forsterite loaded along the orthorhombic a and c axes to pressures from 130 to 165 GPa. Hugoniot states for the two axes are well described by a single curve offset densities 0.15--0.20 g/cm3 lower than earlier data for single-crystal forsterite shocked along the b axis above 100 GPa. Earlier data of Syono et al. <1981a> show marginal support for similar b-axis behavior in the mixed-phase region from 50 to 92 GPa. Thus shocked forsterite is most compressible in the b direction for the mixed-phase and high-pressure regimes (P>50 GPa). These data represent the highest pressures for which shock properties have been observed to depend on crystal orientation. Theoretical Hugoniots for mixed-oxide and perovskite-structure high-pressure assemblages of forsterite calculated from recent experimental data are virtually indentical and agree with the b-axis data. The a- and c-axis data are also consistent with both high-pressure assemblages because uncertainties in equation of state parameters produce a broad range of computed Hugoniots. Our calculated ''average'' Hugoniot is up to 0.13 g/cm3 less dense than the preferred theoretical Hugoniots, in agreement with earlier measurements on dense polycrystalline forsterite. Interpolation between the single-crystal forsterite Hugoniots and Hugoniots for fayalite and Fo45 gives Fo88 Hugoniots bracketing twin Sisters dunite data not previously well fit by systematics. Release paths are much shallower. Hugoniot elastic limits measured for the a and b axes are in good agreement with previous data of Syono et al.; however, the present data for the a axis reveal a triple wave structure: two deformational shock waves as well as the elastic shock, a feature not previously found. The second shock, with amplitude about 9 GPa and a shock temperature of about 350¿K, could perhaps be explained by the forsterite α→β or &ggr; phase transformation.