Persistent photocurrents were observed in semi-insulating GaAs long after the low temperature illumination with monochromatic 0.7—1.8 eV photons was terminated. These excess conductances exceed the equilibrium dark conductances up to a factor 104. We have found that the magnitude of this effect is dependent on the photosensitivity of the material, which is influenced by charge trapping effects. For various photosensitivity stages the concentration of charge trapped in deep traps was determined by measuring thermally stimulated current. Good correlation between the persistent photocurrents and the total concentration of trapped charge was found. The increase of magnitude of persistant photocurrents is interpreted as the increase of lifetime of optically created free hles due to trapping of electrons, which therefore are not available for recombination.