In order to show the type and severity of gross and histopathologic lesions induce by vaccinal strain (serotype A1) of Pateurella mulocida, ten four-week-old SPF chickens were inoculated intramuscularly with 75 cfu of ( 0.5 ml of 10-7 dilution) bacterium. All birds died in less than 16 hours. No prominent gross lesions were observed in different organs. In microscopic examination, the most common histopathological findings were as congestion and hemorrhage. Moreover, large amount of viscid mucus were observed in digestive tracts.