Summerby Swamp, about eight miles north of the Straits of Mackinac and near Moran, has an unusual floristic composition. Mogens C. Nielsen visited there in June 1965 and collected a subarctic composite, Erigeron hyssopifolius, which had not been found in Michigan since its first collection in 1890 by Farwell in Keeweenaw County. Edward G. Voss, Frederick and Roberta Case, and Peter Harley in June 1977 found Empetrum nigrum (black crowberry), which has an arctic, circumpolar range, as well as Primula mistassinica (bird’s-eye primrose), Pinguicula vulgaris (common butterwort), and Juniperus horizontalis (creeping juniper), all with a predominantly lakeshore distribution in the Great Lakes region. This paper reports on a study conducted primarily during the summer of 1980 and giving particular consideration to species of boreal and lakeshore affinities.