The occurrence of avian mycobacteriosis in the rook (C. frugilegus) was investigated in the years 1966-1985. A total of 188 birds captured at three localities of the Czech Republic were subjected to patho-morphological examination and to culture examination. Patho-anatomic lesions were observed in five rooks (2.6%), M. avium was isolated from 10 rooks (5.3%). Positive findings occurred in each examined locality. Rook sensitivity to M. avium infection was tested by experimental infections. After intramuscular infection with M. avium suspension macroscopic tuberculosis lesions were found out at the place of puncture, histological lesions in liver and spleen at the beginning of the trial (days 12-35). Mycobacteria were isolated from particular organs and tissues, more frequently at the beginning of the trial. Peroral infection by feed contaminated with liver of tuberculous fowl failed to induce tuberculous lesions, while a proof of M. avium from the organs was the only one after 157 days. The contact of the rook with the tuberculous fowl did not lead to the origin of tuberculous lesions after 238 days nor were the mycobacteria proved in the rook tissues. In comparison with e.g. the house sparrow, tree sparrow and pheasant, the rook is very resistant to M. avium infection according to the results of experimental infections. A relatively frequent occurrence of tuberculous processes or of mycobacteria in the rook captured in the open area is probably conditioned by a possibility of repeated, frequent contacts of the rook with the contaminated environment during its longer life.(ABSTRACT TRUNCATED AT 250 WORDS)