The 1944 year class of lake trout (Salvelinus namaycush) supported a significant sport fishery in South Bay, Manitoulin Island, over the years 1948 to 1950, inclusive, at a time when that species was absent in any numbers in adjoining Lake Huron. The evidence from tagging indicates that this population of lake trout in South Bay was a resident one. These trout reached a length of 20 inches and a weight of 4 pounds as age-group VI. Growth data based on scale reading are presented for age-groups II to VII and confirmed by observations on marked fish. Estimates of the size of the lake trout populations and of the survival of groups marked in different years were made by fin clipping and tagging through the years 1948 to 1951. The population was estimated to consist of some 21,000 fish in 1948 and to have fallen to 600 fish in 1951, the highest mortality rates coming after 1949. Only 3,124 lake trout were recorded as removed by angling during that period and it is believed that the creel census covered more than 90 percent of the fishery. It is suggested that the sea lamprey (Petromyzon marinus) was responsible for the death of most of the remainder of the population and evidence is presented to show that the incidence of scarred fish greatly increased in the years when the estimates of mortality were also high.