In a previous paper (COCHE, 1967), results concerning the salinity tolerance of coho salmon fry were presented. The present study deals with the ability of yearlings to dispose of the chloride they passively absorb when abruptly transferred from fresh to salt water . Coho salmons normally reside one year in the stream where they were born, the majority of them migrating seawards in the spring of their second year . At this time, they average 12 cm in length and important morphological and physiological changes (smoltification) occur, which prepare the juvenile salmon to be successful in their new environment, the ocean (MALIKOVA, 1957). At this stage of their life, they are capable of passing from fresh water to sea-water, approximately 30 p.p.th. total salt content . The general problem of osmoregulation in fish was reviewed by BLACK (1951a and 1957). But very few studies have been published concerning the osmotic regulation of juvenile coho . BLACK (1951 b) experimented with coho and chum fry . Personally (COCHE, 1967), we made a preliminary study of the survival of coho fry, when transferred from fresh water into sea-water solutions of various salinities . Salinity acclimatization was also tested .