A patient with no known immune compromise presented with necrotizing pneumonia and a pleural effusion. Thoracentesis yielded a sanguinopurulent, foul smelling exudate that showed sheets of polymorphonuclear leukocytes and many gram-positive bacilli. A tube thoracostomy was performed and treatment with intravenous penicillin G was begun. Twenty-four hours later a Bacillus species later identified as Bacillus cereus was identified from aerobic cultures. Because the organism was resistant to penicillin, the patient was switched to intravenous chloramphenicol. A second organism was noted to grow slowly under anaerobic conditions and was later identified as Clostridium bifermentans. Despite initial clinical improvement, fever and empyema persisted resulting in a thoracotomy with resection of the infected lung and extensive decortication. Specimens from the lung tissue obtained at surgery also grew both Bacillus cereus and Clostridium bifermentans. Subsequent review of the original pleural fluid smears revealed a degree of pleomorphism that was not initially appreciated. Prompt surgical intervention combined with appropriate antibiotics resulted in a cure of this unusual aerobic/anaerobic infection.