Recent electrical resistivity measurements have shown that the hexagonal form of stoichiometric NiS exhibits an abrupt metal-to-semiconductor transition at 264°K. Neutron diffraction studies have shown that a first-order paramagnetic-to-antiferromagnetic transition also occurs at 264°K. No crystal lattice distortion is observed at the transition nor detected at 4.2°K, suggesting that this may be a transition of the kind considered by Adler and Brooks. The measured magnetic moment at 4.2°K is 1.66+/-0.08 μβ and at 260°K it is 150.+/-0.10 μβ indicating that the sublattice magnetization is within 10% of saturation immediately upon ordering. The neutron data also shown that no more than about 1% of the Ni atoms migrate to tetragonal interstitial sites on warming from 4.2°K to room temperature. Thus, Ni atom migration apparently plays no part in this transition. The powder magnetic susceptibility is 2.24×10-6 emu/g at 300°K and is virtually temperature-independent above the transition. χ increases abruptly at 264°K by about 15% and exhibits some field-cooling effects. Studies on the compounds NiXS, for X=1.01, 0.99, 0.97, and 0.94 show that excess sulfur lowers the transition temperature.