A Rb-Sr whole-rock age of 429 ± 13 m.y. for the Hall Quarry and Cadillac Mountain Granites, Mt. Desert Island, Maine, establishes that plutonism in this area occurred during the Middle Silurian. The age reported is also a minimum age for the Bar Harbor Series of clastic sedimentary rocks, which are intruded by the Cadillac Mountain Granite. Rocks exposed on Flint Island, Pleasant Bay area, Maine, which contain a Middle Silurian brachiopod fauna, are correlated with the Bar Harbor Series. The minimum age of 429 ± 13 m.y. established for the Bar Harbor Series is in good agreement with other published Rb-Sr ages determined from Middle Silurian rocks elsewhere in coastal Maine.