In undoped bulk-grown GaAs single crystals, which show a wide variation of the resistivity, a characteristic dependence of the Hall mobility on the carrier concentration with a pronounced minimum at about1×1010 cm−3is observed. By applying a standard effective medium theory it is shown that this minimum is caused by mesoscopic nonuniformities of the charge carrier concentration and not by increased scattering rates or additional scattering mechanisms as would be the standard interpretation in the case of homogeneous samples. These nonuniformities observed by high-resolution point-contact measurements are connected with the cellular structure of dislocations. ©1997 American Institute of Physics.