Five strains of salmon Salmo salar and a strain of Danish rainbow trout Oncorhynchus mykiss were experimentally infected with the ectoparasite Gyrodactylus salaris (Lærdalselva strain, Norway). All fish were hatchery-reared and the genetic origins were from the East Atlantic: River Conon (Scotland), Storå (western Denmark) and Ätran (western Sweden) and from the Baltic: River Lule and Ume (Sweden). The rainbow trout used were from a Danish fish farm. Three replicate aquaria infested with G. salaris were established containing 10 fish of every strain. The numbers of parasites were assessed on anesthetized fish once a week from week 0 to week 8 and concurrently the location of every parasite on each of twelve regions on the fish was recorded. The mean abundance of G. salaris steadily increased on the East Atlantic Conon, Storå and Ätran strains until the end of the experiment. The mean abundance of parasites on the Baltic Lule and Ume strains initially increased but after a time period of 4-7 weeks, the growth of the parasite infrapopulations decreased. On all species and strains the fins and head had the highest proportion of parasites. Over time, there was a trend to an increasing percentage of parasites on the tail fins of the salmon from the East Atlantic strains and a converse tendency of decreasing percentages on this site was seen on the Lule salmon. The pectoral fins of the Lule salmon had a relatively higher percentage of parasites compared to salmon from the East Atlantic strains. The present study showed that the East Atlantic strains of salmon were highly susceptible to G. salaris whereas the two Baltic salmon strains were able to keep the parasite infection at a low level. The Danish rainbow trout strain showed an even lower susceptibility.