We present low temperature photocurrent (PC) spectra of ordered and disordered GaAs/AlGaAs superlattices (SLs). The investigated disordered SLs were designed to provide both localized and extended states. Our results show a dramatic difference between the PC of ordered and disordered SLs. In the ordered case, the PC is observed to decrease abruptly on increasing the applied voltage V, as expected. In contrast, the total PC signal in the disordered SLs increases in steps as a function of the electric field. Our measurements indicate a transition from a conduction regime to Stark localization in the ordered case, and from the Anderson localization to a conductive regime in the disordered ones. Numerical results obtained by the transfer matrix method indicate the weak dependence and the red shift with V of the disordered SL miniband as the causes of the observed behavior. (C) 1999 Elsevier Science Ltd. All rights reserved.