We show that a photoconductivity relaxation can strongly influence the magnitude of the ac current induced in a bipolar photoconductor by a vibrating pattern of interference fringes. This occurs for low pattern vibration frequencies if the time of the photoconductivity relaxation is much greater than the dielectric relaxation time. The features predicted by the theory were observed in semi‐insulating GaAs illuminated by a He–Ne laser (&lgr;=633 nm) for vibration frequencies lower than 20 kHz. ©1995 American Institute of Physics.