We report a case of pulmonary infection by Mycobacterium avium complex revealing in a 21 years old patient a cystic fibrosis heterozygous for the delta F 508 deletion. The role of this bacteria in the lung infection is suggested by repeated isolation. This infection caused bronchitis, hypoxemia and pulmonary nodules at CT scan. The clinical and radiological signs improved after treatment with four antibiotics whereas only clarithromycin showed in vitro activity. An obstructive disease due to allergic bronchopulmonary aspergillosis developed and was controlled by steroid therapy.