Published information on the winter diet of landlocked Atlantic salmon Salmo salar has been extremely limited. Analyses of winter foods of 484 Atlantic salmon from four Maine lakes indicated that the primary food, when available, was rainbow smelt Osmerus mordax. Agespecific length and weight of Atlantic salmon from Moosehead Lake decreased from 1985 to 1987, which we attributed to reduced abundance or availability of rainbow smelt. In Schoodic Lake, where rainbow smelt were scarce, Atlantic salmon fed on insect larvae or other fishes, and growth was poor. Our data indicate that the availability of rainbow smelt in winter may play a prominent role in determining both the growth of landlocked Atlantic salmon and the ability of managers to achieve objectives established for the fishery.