Elastic properties of ZnO polymorphs: B4 (wurtzite), B3 (zinc-blende), B1 (rocksalt) phases, and yet experimentally undiscovered B2 (CsCl) phase were computed using the first-principles plane-wave pseudo-potential method in the scheme of density functional theory. The results obtained for the most stable B4 structure are in excellent agreement with the most recent experimental data obtained from single-crystal measurements with contactless Brillouin scattering spectroscopy. It is demonstrated that the hypothetical B2 phase of ZnO is mechanically unstable in ambient conditions.