The minimum critical core size for the Southwest Experimental Fast Oxide Reactor was predicted prior to the initial fuel loading using the results of a critical mockup in ZPR-3. The predicted minimum critical loading was 519 PuO2-UO2 fuel rods containing 285 kg of fissile (239Pu + 241Pu) plutonium, with 1 beryllium oxide tightener rod for each 6 fuel rods. The actual minimum critical loading was equivalent to a core containing 518 standard fuel rods and the nominal 6-to-1 ratio of fuel-to-tightener rods. The calculations used in the prediction are described and the agreement between experiment and calculation is discussed.