Mg‐doped AlxGa1−xAs crystals with Al concentration 0⩽x⩽0.8 have been grown by the conventional LPE method. Doping characteristics of Mg and electrical properties of the crystals are studied using Hall measurements in the temperature range between 77 and 295 K. The results revealed are as follows. Magnesium works as an acceptor in AlxGa1−xAs. Its distribution coefficient is constant and about 0.6 for AlxGa1−xAs withx⩾0.1. Hole concentrations up to 1018cm−3are easily obtained for a highxof 0.65. The acceptor energy levelEiincreases from 18 meV in GaAs to 40 meV in Al0.8Ga0.2As. Thexdependence ofEican be explained by the difference of effective hole masses and dielectric constants between GaAs and AlAs. Hole concentration, mobility, and resistivity, as well as their temperature dependences, are presented for variousxvalues. These data show that Mg is a suitable acceptor in AlxGa1−xAs particularly with highxvalues.