A micromagnetic model was developed to calculate the maximum grain size for which an ideal parallelepiped-shaped particle can have a metastable single-domain (SD) structure. For mathematical tractability, the micromagnetic equations were described only by one-dimensional deviations from a uniform SD state. It was found that for cubic-shaped magnetite particles, the metastability threshold, acr was approximately 0.14 micrometers, well above the equilibrium threshold size for the single-domain to two-domain transition. The metastability threshold size also increased rapidly with particle elongation and for a length: width ratio greater then 2.6, magnetite grains of all sizes could theoretically support a metastable single-domain state. ¿ American Geophysical Union 1988