The relative virulence was determined for 14 species of aspergilli, by inoculating normal mice intravenously with graded doses of spores. Eleven were found to possess some degree of virulence, whereas three others were avirulent. Members of the Aspergillus flavus group were the only species that consistently killed mice with doses as low as 10(4) viable spores. When the in vivo fate of spores was compared for a virulent and an avirulent strain of Aspergillus, spores of the latter were cleared rapidly from the liver and spleen but grew in the kidneys and brain, producing progressive disease. Mice which inhaled spores did not succumb, but macrophages washed from their lungs contained spores. A relationship of virulence to spore characteristics such as germination time, size, shape, and external markings could not be established. Virulence could not be related to aflatoxin production inasmuch as at least one virulent strain did not produce aflatoxin in vitro.