Gross and microscopic pathology caused by an atypical strain of Pasteurella gallinarum (Fresno strain) was compared in chickens with that caused by the American Type Culture Collection type strain. Ten 21-day-old broiler chickens were inoculated intranasally with 10(7) colony forming units or intramuscularly with 10(5) colony forming units of either strain. The birds were killed 7 days later, and gross and microscopic lesions were studied. Grossly, there was extensive white discoloration of pectoral muscles with mild fibrinous exudate in birds inoculated intramuscularly with the Fresno strain of P. gallinarum. Most of these birds also had severe fibrinous exudation over the heart, the capsule of the liver, the air sac, and in the hock joints. Microscopically, there was severe chronic pyogranulomatous airsacculitis, pericarditis, perihepatitis, myositis, synovitis, and granulomatous pneumonia. One bird had severe acute multifocal hepatitis. From this study, it is evident that the Fresno strain of P. gallinarum was more pathogenic than the American Type Culture Collection type strain when given intramuscularly.