This study represents the first validation of the scale method of age estimation for lake whitefish ('Coregonus clupeaformis') within the Laurentian Great Lakes. Transition ridges (annuli) form annually on the scales of Georgian Bay lake whitefish. Marginal increment analysis indicates that transition ridges form bi-annually in the spring and fall on the scales of main basin lake whitefish and that multiple mechanisms may be responsible for their formation. Transition ridges form in the fall and spawning is implicated as the ecological mechanism of formation for Georgian Bay lake whitefish. We can now confidently assign ages to Georgian Bay lake whitefish in the 2-14 age range, knowing that the scale method is valid. However, validation must be an ongoing process and changes in life history or the environment which mediate changes in growth will necessitate a reevaluation of the scale method of age estimation for lake whitefish in Georgian Bay.