A Study was carried Out to obtain information about the tolerance of Caligus rogercresseyi to salinity. Samples of C. rogercresscyi were collected from Atlantic salmon (Salmo salar) in four areas of southern of Chile (Lat 42 degrees S), which were exposed to different concentrations of salinity. From the results obtained it was evident that Caligus displayed sensitivity to salinity lower than 20 parts per thousand. However differences of tolerance were observed in the parasites collected from Puerto Montt-Castro region compared with parasites collected from Quellon and Hornopiren, on these latter sites freshwater had a major influence and reduced the intensity of infection.