Abstract: The present invention relates to a live vaccine against Pasteurella multocida, for application of poultry by injection, per os or as aerosol, containing an attenuated non-virulent genetically stable strain of Pasteurella multocida, to a process for producing an attenuated genetically stable non-virulent strain of Pasteurella multocida, which comprises cultivating a virulent field strain, selecting after 18 hours at 37.degree. C. colonies of uniform size, of about 4 to 5 mm diameter, suspending these in a pH 5.5 buffer, subjecting same to a mutagen at a concentration causing a mortality of 90-99% after 30 minutes at 37.degree. C., centrifuging the microorganisms, washing same, plating same and selecting colonies of not more than 2.5 to 3 mm diameter after 18 hours at 37.degree. C.