A 29-year-old patient with AIDS was hospitalized with weight loss, fever and cough. Mycobacterial cultures from sputum, blood and bronchoalveolar lavage became positive after 3 weeks' incubation. When using a DNA probe for identification of Mycobacterium tuberculosis complex, a weakly positive signal was obtained. Tuberculosis was suspected and treatment was started with isoniazid, ethambutol and ciprofloxacin. Sequencing of the gene of the 16S rRNA, however, identified the isolates as belonging to a new, slow-growing atypical mycobacterial species, Mycobacterium celatum (M. celatum). Treatment was modified to take into account the previously described primary resistance of M. celatum to antituberculous drugs, whereupon the patient improved.