High-pressure single-crystal X-ray diffraction experiments with natural ferrosilite Fs 82 (Fe 2+ 0.82 Mg 0.16-Al 0.01 Ca 0.01)(Si 0.99 Al 0.01)O 3 orthopyroxene (opx) reveal that at ambient temperature the sample does not transform to the clinopyroxene (cpx) structure, as reported earlier for a synthetic Fs 100 end-member (Hugh-Jones et al., 1996), but instead undergoes a series of two polymorphic transitions, first above 10.1(1) GPa, to the monoclinic P2 1 /c phase b-opx (distinctly different from both P2 1 /c and C2/c cpx), also observed in natural enstatite (Zhang et al., 2012), and then, above 12.3(1) GPa to a high-pressure ortho-rhombic Pbca phase c-opx, predicted for MgSiO 3 by atomistic simulations (Jahn, 2008). The structures of phases a, b and c have been determined from the single-crystal data at pressures of 2.3(1), 11.1(1), and 14.6(1) GPa, respectively. The two new high-pressure transitions, very similar in their character to the P2 1 /c–C2/c transformation of cpx, make opx approximately as dense as cpx above 12.3(1) GPa and signif-icantly change the elastic anisotropy of the crystal, with the [1 0 0] direction becoming almost twice as stiff as in the ambient a-opx phase. Both transformations involve mainly tetrahedral rotation, are revers-ible and are not expected to leave microstructural evidence that could be used as a geobarometric proxy. The high Fe 2+ content in Fs 82 shifts the a–b transition to slightly lower pressure, compared to MgSiO3 , and has a very dramatic effect on reducing the (meta) stability range of the b-phase.