Micromorphological changes seen in the respiratory tract of pigeons with contact infection with Aspergillus fumigatus were secondary to fungemia and mycotoxemia. The micromorpholo gical changes, such as chronic granuimatus lesions, were due to the reaction of the tissue against the presence of the fungus. Lymphoid cell proliferation was due to immune defense against the fungal infection. These observations indicate the importance of different routes of Aspergillus fumigatus infections in this species of birds.