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The phocine distemper outbreak that killed more than 18 000 harbour seals (Phoca vitulina) and a number of grey seals (Halichoerus grypus) in the North and Baltic Seas in 1988 is described. The high mortality was probably due to a highly virulent agent being introduced into a naive population with no specific immunity to the infectious agent. Organochlorine pollution and crowding of seals at haul-out sites may have exacerbated the severity of the outbreak. Migrating harp seals (Phoca groenlandica) may have been vectors of the disease. The disease could have spread to Danish mink farms. There are no epidemiological links between the outbreak in Europe and outbreaks in Baikal seals (Phoca sibirica) and Arctic sled dogs in 1987-88.