21 Gyrodactylus species, described from salmonids are presented, arranged in six Gyrodactylus-groups. Findings of the species in fish farms and/or natural waters are summarized. With data from Gyrodactylus species in natural waters in Scandinavia and in the Baltic as a background, biological, ecological and behavioural observations in natural waters of G. salaris Malmberg, 1957 and G. derjavinisensu Malmberg et Malmberg (1987) in Swedish and Norwegian rivers with wild salmon are presented. The unique viviparity, asexual and sexual reproduction and the reproductivity in Gyrodactylus are emphasized. Gyrodactylosis on salmonids in natural waters and fish farms is dealt with, e.g. the G. salaris gyrodactylosis in Norwegian rivers and in Swedish and Danish salmonid farms. The ultrastructure of wounds caused by G. salaris and results from Norwegian and Canadian experiments with Gyrodactylus species on salmonids are presented. The natural geographical distribution of salmonids, distribution by man, and economics, including culturing of salmonid species are reviewed.