A 25-yr-old male freshwater crocodile (Crocodylus johnstoni) was diagnosed with pulmonary mycobacteriosis caused by Mycobacterium szulgai. Necropsy revealed fibrinous exudate in the right pleural cavity and white miliary nodules in the right lung lobe. Histopathologic examination revealed well-demarcated granulomas consisting of multinucleated giant cells and epithelioid cells surrounded by fibrous connective tissue. Atypically, lymphocytes had accumulated in the outer region of fibrous connective tissue. Mycobacterial infection was confirmed by nested polymerase chain reaction targeting the hsp65 gene and by Fite's method for detection of acid-fast bacilli within formalin-fixed, paraffin-embedded lung tissue. Sequence analysis of the DNA amplicon revealed that the species of mycobacterium shared 98% homology with the gene encoding the hsp65 gene of M. szulgai. This is the first report of M. szulgai as the causative agent of mycobacteriosis in a reptile.