Actinomycosis is a disease caused by Actinomyces israelii, a gram-positive, anaerobic microorganism which usually lacks pathogenic activity. Its most common clinical expression is infection of the skin of the face and neck, since the microorganism is a frequent resident of the nose and throat. Infectious behavior is extremely rare in other organs. We present the case of an adult patient with respiratory manifestations which evolved over a period of three months, who was diagnosed with Pulmonary Actinomycosis after lobectomy performed due to an initial clinical suspicion of malignancy (Acta Med Colomb 2010; 35: 132-134).