A theoretical model of grain size dependence of domain transitions in magnetite is presented. This domain model is used to predict the grain size range for pseudosingle domain (PSD) behavior in magnetite. For cubic magnetite particles, the single domain (SD) to two-domain (TD) transition occurs at ~800 ± 200A, the TD-three-domain transition occurs at ~l500A, and the PSD-“true” multidomain (MD) transition occurs at ~8 μm. These results are in fair agreement with experimental results within a factor of 2. Our results indicate that the grain size range for the canonical case of a PSD grain (a TD grain with a single 1800 wall) or “PSARK” (subdomain magnetic moment) ranges from ~700A for cubic particles of magnetite to ~3.2 μrn for a length: width ratio of 5: 1. Comparison with grain size distributions of magnetite grains in igneous rocks suggest that the PSARK can be the major contributor to the remanence of these grains.