The occurrence of avian mycobacteriosis and sensitivity to M. avium were investigated in the laughing gull in this paper. The patho-morphological and culture examination of 35 individuals of the laughing gull caught at four different localities of the Czech Republic did not reveal any tuberculous lesions nor did it enable to isolate mycobacteria. After experimental intramuscular infection by suspension of M. avium macroscopic tuberculous lesions were found at the spot of puncture in 35 days, in liver and spleen in 70 days since the beginning of infection. In the simultaneously infected domestic fowl the lesions were determined at the spot of puncture, in spleen and liver in 35 days. Mycobacteria were isolated from several organs and tissues in both species in 35 days after infection. Tuberculous lesions were found in liver, spleen and intestines of the laughing gull in 118 days since peroral infection by feed contaminated with the liver of tuberculous fowl. This finding was identical in spleen of the domestic fowl only in 118 days. Mycobacteria were demonstrated in both species in 118 days, in liver, spleen, intestine and/or brain in the laughing gull, while in spleen only in the domestic fowl. Intestinal tuberculosis was determined in the laughing gull in 118 days after infection. The results of experimental infections show that the sensitivity of the laughing gull to M. avium is the same as in the domestic fowl. The occurrence of tuberculosis in the laughing gull is apparently depending upon the local epizootological situation, particularly upon the occurrence of M. avium sources and their accessibility.(ABSTRACT TRUNCATED AT 250 WORDS)