A physical model is proposed for the photoplastic effect in ZnO, i.e., the reversible increase in flow strength upon illumination. The model is based on the assumption of Coulomb interaction between dislocations, which are negatively charged, and photoionized traps, which contain excess positive charge. Negative charge on dislocations has been observed in silicon, germanium, cadmium sulfide, and other semiconductors. The excess positive charge appears when interstitial Zn+atoms are doubly ionized to Zn2+under the influence of light. Several measurements were made of the photoplastic effect which confirmed the proposed model. First it was found that the photoplastic effect depended on the deformation rate so that at a high strain rate, 5×10−2sec−1, the effect disappeared, presumably because of the high propagation velocity of the dislocations, which did not allow sufficient time for the photoionization to grow. At lower strain rates, 5×10−4sec−1, a repeated yielding occurred, also explained by the model. In a second experiment photoconductivity at high dislocation densities was studied. An analysis of the photoconductivity data made it possible to predict the saturation illumination intensity and the time constant of the photoplastic effect. Agreement with the observed values was found. In a third experiment it was found that the flow stress, with and without illumination, depended on the square root of the density of interstitial zinc atoms.