We have studied the in-plane magnetic anisotropy of epitaxial MnSb (1<(1)overbar>01) films grown on GaAs (001) by molecular beam epitaxy. The MnSb films were grown on (2x4) and (4x6) reconstructed GaAs surfaces at 250 and 50 degrees C. At 250 degrees C, the films showed a strong twofold in-plane magnetic anisotropy independent of the GaAs surface reconstruction. In contrast, at 50 degrees C, the in-plane anisotropy appeared only on the (2x4) reconstructed surface. The anisotropic crystallographic domain structure of the MnSb films is thought to cause the magnetic anisotropy. The anisotropic domain formation is explained by the different chemisorption of the Mn adatom on the GaAs surface as a function of the termination. (C) 1998 American Institute of Physics. [S0003-6951(98)01348-5].