A study of the mobility &mgr; of Se‐dopedn+‐GaAs grown by metalorganic chemical vapor deposition is presented. A significant decrease in &mgr; is observed forn>1×1018cm−3, which is a general characteristic ofn+‐GaAs. Previous explanations that the low values of &mgr; are the result of autocompensation by the dopant are unsatisfactory in view of the universality of the decline in &mgr;. A new formula is derived for the ionized impurity mobility &mgr;Ifor degenerately doped material which accurately predicts the experimental &mgr; using no compensation and no adjustable parameters. The formula takes into account the increase of the effective massm* due to nonparabolicity of the conduction band and due to distortion of the band by the donor atoms. For degenerate material, &mgr;Iis inversely proportional to the square ofm* at the Fermi energyEF. For uncompensated GaAs withn=1×1019cm−3,m* atEFis 2.4 timesm* for pure GaAs, and &mgr; is only 1000 cm2/V s. Previous theories, which use the smaller optical effective massm@B|optin place ofm*, predict higher mobilities and temperature variations of &mgr; forn+‐GaAs. The new formula correctly predicts that &mgr; does not change with temperature whenn>2×1018cm−3.