The influence of a weak magnetic field on the behavior of structural defects in Si and GaAs has been studied by the methods of photoconductivity, magnetic susceptibility and microhardness. The impurity photoconductivity of mechanically damaged GaAs and the remarkable reduction of induced photoconductivity after exposition in magnetic field of 0.3 T were observed.. These effects are much less pronounced for Si samples. The potential fluctuations created by charge defects are invoked to explain the magnetic field influence on structural defects. (C) 2003 Elsevier B.V. All rights reserved.