In 1996, an epizootic occurred among wintering and breeding common eiders Somateria mollissima in southwest Kattegat, Denmark, causing the death of at least 900 birds during late winter, and of a total of 3,146 females in five local breeding colonies, corresponding to 35–95% of the females present within the single colonies. The cause of death was related to a bacterial infection by Pasteurella multocida isolated from all examined eiders collected on wintering and breeding grounds. This is the first documented incidence of pasteurellosis in Scandinavia. Based on knowledge of the phenology and winter distribution of eiders, the temporal occurrence of the disease suggests that apparently healthy birds acted as carriers of the disease bringing it from the wintering grounds to the breeding colonies.