The Hart’s-tongue, Phyllitis scolopendrium, has been noted for its circumboreal and North American disjunct distributions. On this continent P. scolopendrium var. americana Fern. is know to occur in Ontario, New York, Tennessee, and Alabama. By far the majority of the Hart’s-tongue sites are associated with the limestones and dolomites of the Niagara escarpment. This geological formation can be traced from central New York westward into Ontario, where it turns northwestward near the head of Lake Ontario, and through the Bruce Peninsula and Manitoulin Island in Lake Huron, into the upper peninsula of Michigan. From there it arcs southwestward through Wisconsin’s Door Peninsula to the east of Green Bay and disappears to the south. This paper will deal principally with the northernmost American Hart’s-tongue colonies, those in upper Michigan, and with some ideas concerning the factors determining its distribution in that region.