This is a bacteriological report of a case of a primary acute inflammation of the serous membranes (peritoneum, pericardium, pleura). A bacteria apparently not yet described in the litterature was isolated. it is here described as Bacillus serositidis, and it was classificated according to the systhem of the American Bacteriologists Society. Bacillus serositidis exhibits the same morphology either in the pus, smears and sections of the serous membranes or in the culture media. The patient, evidently, died from a fibrino-purulent inflammation of the serous membranes. This acute inflammatory process was probably caused by the bacillus found in the smears and sections as no other injury could be demonstrated.