Abstract.Gyrodactylus salariswas probably introduced into the River Vefsna, a large salmon river in northern Norway, by stocking of Atlantic salmon smolts from infected hatcheries in 1975 and 1977. An outbreak ofG. salarisamong salmon parr was observed in 1978. During the following 2 years, the parasite spread throughout the entire watercourse. There was a rapid rise in prevalence rate and degree of infection. The density of salmon parr decreased from a high level before theGyrodacrylusoutbreak to close to zero after the outbreak. Salmon parr mortality is probably associated with an increase in the parasite burden. Infected fish appear to survive for slightly more than 1 year and this time period is sufficient for transmission of the parasite to the next year class. The total catch of salmon ascending the river has also greatly decreased in recent years.