Among the lake trout, Salvelinus namaycush (Walbaum), of eastern Lake Superior are some which, instead of spawning on reefs and shoals in the lake, enter certain rivers to spawn. Tagging and recovery studies were conducted on such fish seined from the Montreal and Dog rivers during the period 1951 through 1955. Male trout in Montreal River were found to mature at age 7 years when their average length was 25 inches. Females matured a year later when their average length was 28.5 inches. Trout in Dog River matured at the same ages but at lengths shorter by 3 to 4 inches, apparently because of slower growth during early life. Captures of the tagged trout in Lake Superior by sport and commercial fishermen showed that they frequented depths from 10 to 30 fathoms. Most of the trout recaptured were taken within 30 miles of the river in which they spawned. High recapture rates in the succeeding spawning season in the rivers at which they were tagged suggest that most of the trout return to the same river annually to spawn, and that the populations associated with each spawning run are discrete. The mortality rate of fish in the spawning run increased during the study period and reached an estimated 90 percent in both populations for the year 1954 to 1955. Spawning populations in both rivers declined from over 2,000 fish in 1952 to only a few fish in 1955. Catch of trout by sport and commercial fishermen was light, and was estimated to be less than 5 percent of the population each year. Incidence of scars caused by attacks of sea lamprey, Petromyzon marinus, greatly increased during the years when the trout population declined. It appears likely that the rapid decline of these populations of stream-spawning lake trout was caused by the increase of sea lampreys in Lake Superior.