Pyothorax (thoracic empyema) in two females 4 and 3.5-4-month-old cats are reported. The cats were referred to Small Animal Hospital, School of Veterinary Medicine, University of Tehran, Tehran, Iran, with the clinical signs and symptoms of acute fever, severe respiratory distress, anorexia and malaise. The clinical and radiological examinations pointed to an accumulation of fluid within the thoracic cavity. Because of the severity of the disease the animals died without obtaining any treatment. In post mortem examination, a bilateral pyothorax was diagnosed. Both Pasteurella multocida and Nocardia asteroides and Pasteurella multocida alone were isolated from case 2 and case 1, respectively.