Monoclinic pyrrhotite (Fe7S8 with 4C superstructure) was magnetically separated from a massive pyrrhotitic ore for preparation of nine grain-size fractions ranging from ~80 &mgr;m (large multidomain grains) to <3 &mgr;m (single-domain grains). Grains smaller than 100 &mgr;m were found to be magnetically hard with coercive forces ranging from 135 oe for 83 &mgr;m grains to 920 oe for <3 &mgr;m grains. For grain sizes between 83 &mgr;m and 7 &mgr;m the coercive force is given by Hc∝d-0.7. Variations in hysteresis properties with grain size appear to be gradual, with no evidence of sudden changes associated with domain structure transitions.