This paper reports on a group of strains of Mycobacterium avium-intracellulare recently isolated from various bird species. The strains in question could not be integrated into the known 28 serovars. The host spectrum includes birds of several avian orders. Clinical, serological, pathoanatomical, and histopathological results are being discussed. During observations of the clinical course in some infected birds, it could be shown that the excretion of the agent could cease for several months. The diagnosis of the new group of serovars was possible only by culture. Antibodies of several serovars demonstrated in life birds did not correspond with the isolates. Therefore, these antibodies were interpreted as serological crossreactions.