A total of 253 birds were investigated to determine the presence of mycobacteria. Scrapings from various internal organs were stained according to Ziehl-Neelsen, and acid-fast bacilli were found in 26 birds (10.2%). Cultivation of mycobacteria was attempted from 22 livers, 12 spleens, 14 kidneys, 12 lungs, and 9 intestines from these 26 birds. Each sample was first decontaminated using a modified sodium dodecyl sulfate method, and three media were inoculated (Bactec 12 B; Löwenstein-Jensen, Herrold). Using a Polymerase-Chain-Reaction-Restriction-Enzyme-Analysis (PRA), M. genavense could be identified in 19 of 26 birds (73%). M. avium could be isolated from three birds and M. fortuitum from one bird. In total, mycobacteriosis was the primary diagnosis made in 24 of 26 birds (92%). A presumptive diagnosis of mycobacteriosis was already made macroscopically in 14 of these birds. In the remaining 10 cases, bacteriological and histological investigations with specific staining methods were necessary for diagnosis. Several different histological changes were found. We observed individual macrophages as well as epitheloid cell proliferation, with variable, relatively mild inflammatory and fibrous reactions. We could not find any correlation between infection with a mycobacterium species and a specific tissue reaction pattern. This report demonstrates that M. genavense is an important cause of avian mycobacteriosis especially in pet birds.