We observe that, when ZnIn2S4monocrystals at a temperature below 170°K are illuminated by light of the gap energy, the dark resistance, after illumination, is lowered by about nine orders of magnitude. This high‐conductivity state persists until the crystals are either (a) warmed to room temperature, (b) exposed to light of a definite energy less than the gap width, or (c) placed in an electric field. We propose a level 1.76 eV above the valence band, with a repulsive barrier of 0.11 eV, which an electron must penetrate to enter this level and recombine.