Two outbreaks of fowl cholera in the avifauna in Denmark, affecting primarily eiders but also cormorants, gulls and oyster-catchers were shown to be caused by the same clone of Pasteurella multocida ssp. multocida by restriction enzyme analysis (REA) and ribotyping, using the enzymes HpaII and HhaI and phenotypic characterization. This observation indicated spread by migratory birds. It was shown that the outbreak clone was closely related to isolates of Pasteurella multocida ssp. multocida obtained from back-yard poultry in Denmark, including chickens, pheasants, turkeys and ducks. The only detectable difference between the outbreak clone and some of these strains concerned the size of one fragment. These results indicate a possible exchange of P. multocida ssp. multocida between populations of wild birds and back-yard poultry. Among the DNA fingerprinting methods used, restriction enzyme analysis offered the highest discrimination among thirty strains obtained from back-yard poultry. The restriction enzymes HpaII and HhaI generated almost the same number of profile types, 17 and 15 respectively, but only HpaII differentiated the outbreak clone from the group of closely related strains isolated from back-yard poultry. Ribotyping, using the same enzymes, resulted in 12 and 10 different profile types, respectively. The outbreak isolates did not harbour any plasmids, while six out of the 30 strains originating from back-yard poultry (20%) carried a cryptic plasmid of approximately 3.4 kb.