Undoped liquid encapsulated Czochralski grown GaAs crystals with a transition from semi-insulating to medium-resistivity behavior show unusual low values of the Hall mobility at 300 K in this transition region. Moreover, in samples of this region an anomalous temperature dependence of&mgr;Hcharacterized by an increase of&mgr;Hwith increasing temperature forT<400 Kis observed. By model calculations using a standard effective medium theory it is shown that this anomalous behavior of the Hall mobility is due to the existence of mesoscopic electrical nonuniformities connected with the cellular structure of dislocations. ©1997 American Institute of Physics.