A 62-year-old man was admitted with the complaints of chronic sputum, dyspnea, and general weakness. Chest X-ray and computed tomographic films disclosed severe mediastinal shift and left lung collapse due to the accumulation of fluid and gas in the left pleural space. A puncture of the thoracic cavity yielded a milk-coffee-like purulent pleural effusion with stool odor, suggesting pyothorax with pneumothorax or broncho-pleural fistula. Chest tube drainage was performed. The elimination of gas was transient; subsequently, no air leaks were observed during deep breathing, suggesting the absence of pneumothorax and broncho-pleural fistula. An anaerobic culture of pleural effusion was prepared and a Bacteroides species was isolated. These clinical findings indicated that the intrathoracic gas could have been produced by anerobic bacilli. Systemic antibiotic chemotherapy with chest tube drainage achieved recovery. The production of gas in focal lesions is one noted symptom of anaerobic bacillus infection. However, to our knowledge, cases of anaerobic bacillus pyothorax generating large volumes of intrathoracic gas are rare.