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This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of 2.450. 15eV, whose pressure coefficient is very small. At higher photon energies, a direct transition is observed (4.6 eV at 10 GPa), with a positive pressure coefficient (around 40 3 meV/GPa between 5 and 19 GPa). These results are interpreted on the basis of first-principles electronic band structure calculations.© 2003 American Institute of Physics.