Biological investigations connected with hydropower development of rivers in Northern Norway in 1975-79 coincidently led to the discovery of the ectoparasite Gyrodactylus salaris as a serious problem for Atlantic salmon (Salmo salar L.) populations in Norway. Regional research of Atlantic salmon rivers was initiated by a Gyrodactylus committee in 1980 and has been continued each year. G. salaris is most probably a recent introduction to Norway, and is now reported in 34 rivers and about 35 hatcheries. The distribution of G. salaris in Norwegian rivers is connected with stocking of fish from infected salmon hatcheries. Populations of salmon parr have been severely reduced in infected rivers. In recent years catches of ascending salmon in these rivers have also declined. A primary goal of Norwegian fishery authorities is to prevent the further spread of G. salaris and to exterminate the parasite in as many infected rivers and hatcheries as possible. Extermination has been accomplished in the river Vikja in the Sognefjord by using rotenone treatment in 1981-82. G. salaris has not been found in this river following treatment.