Thoracic actinomycosis was identified in a 14 year old girl. The disease is infrequent in children and diagnosis is difficult. The occurrence of a protracted pulmonary lesion, together with osteomyelitis of several ribs and by fistulae formation in the chest wall, with production of characteristic "sulfur granules", were the key elements to suspect the right diagnosis, which was confirmed by a bacteriological and histological studies. Treatment with high doses of intravenous penicillin, 3 million U four times a day for 4 weeks, followed by long term oral therapy with the same drug was done, with remission of pulmonary, osseous and cutaneous lesions.