Single-crystal CoO films, 3 to 30 μ thick, were grown epitaxially on MgO by halide decomposition. Optical, x-ray, and magnetic torque measurements were made on the as-grown samples, i.e., with the CoO attached to the MgO. Films with (001) and (111) orientations had a spiral growth structure. Twinning in the CoO films below the Néel temperature was investigated by optical and x-ray techniques. Antiferromagnetic twin domains were observed in (001) CoO films at 77°K by transmitted polarized light. The tetragonal distortion in CoO was measured down to 83°K and had bulk values with a c/a ratio of 0.9880 at 83°K. Magnetic torque measurements were made from 77° to 296°K in fields up to 22 kOe. The symmetry and amplitude of the torque varied when the sample was cooled from room temperature in a field. Cooling in a field altered the spin axes distribution to favor those spin axes most perpendicular to the cooling field direction. The torque data for the (111) orientation were consistent with a single- and a multi-spin-axis model. However, the torque data for the (001) orientation were consistent only with a single-spin-axis model. © 1966 The American Institute of Physics.