The field dependence of the magnetic susceptibility of MnO powder has been studied in continuous fields up to 50 kOe, between 4.2° and 200°K. Each isotherm is measured from a virgin magnetic state to avoid hysteresis and viscosity complications. Above the antiferromagnetic Ne´el point, the susceptibility is field independent with a maximum value of (83±1)×10−6emu/g at 122°K. At 4.2°K, the susceptibility saturates with field at a value of 79×10−6emu/g. This is identified as the transverse susceptibility. It drops slightly with temperature to a broad minimum at 55°K. An attempt to analyze the field dependence using the model of Keffer and O'Sullivan suggests that the weak in‐plane anisotropy,K2, has orthorhombic symmetry. This model is rejected because it ignores domain wall displacement. A rough estimate ofK2at 4.2°K is 9×103erg/cc, assuming trigonal symmetry.