Expts. involving marking or tagging of anadromous salmon or trout are reviewed and summarized. Two alternative hypotheses are viewed in the light of these expts. and of studies of local stocks of salmon. The hypothesis most compatible with the available facts is the following: Anadromous salmon or trout hatched and reared in a particular region will, when seeking fresh water to spawn, seek out and return in the great majority of cases to the same region, even from considerable distances. The mechanism of the return is briefly discussed. 88 references.