Groups of 3-week-old turkey poults were exposed to aerosols containing spores of either Aspergillus fumigatus or aflatoxigenic or nonaflatoxigenic strains of Aspergillus flavus. Approximately 5 X 10(5) spores of A. fumigatus/gm of lung tissue killed about 50% of the turkey poults with A. funigatus, whereas doses approximating this level with either of the A. flavus strains did not cause any mortalities for up to 8 weeks postexposure. Fungi were isolated from lung tissue of 60% of the birds surviving for 8 weeks after exposure to spores of A fumigatus, but A. flavus was isolated from lung tissue from only 20% of the birds 8 weeks after exposure to either A. flavus strain. Almost all birds exposed to A. fumigatus developed precipitating antibodies, whereas none of the birds exposed to A. flavus developed precipitating antibodies. The lesions in the lung tissue from birds exposed to A. fumigatus were larger, were more numerous, and involved a greater percentage of tissue than was observed in lung tissue from birds exposed to A. flavus. There were no apparent differences in pathogenicity between the A. flavus strains. Aflatoxin was not detected in tissues from birds exposed to the aflatoxigenic strain of A. flavus, and there were no signs of aflatoxigenic when sections of liver from these birds were examined histopathologically.