Fifteen cases of thoracic actinomycosis have been reviewed at the University of Cincinnati Medical Center and their roentgenograms analyzed. Although the roentgen manifestations of the disease are for the most part nonspecific, the following features should suggest the diagnosis: 1. Extension of a pulmonary lesion through the thoracic wall. 2. Wavy periostitis or frank destruction of ribs associated with contiguous chronic pulmonary involvement. 3. Penetration of a pulmonary lesion through an interlobar fissure. 4. A characteristic pattern of vertebral destruction. Any chronic pulmonary infiltrate, especially if accompanied by cavitation or pleural involvement, is suspect.