There is often more genetic diversity in a given ecosystem than is represented by current taxonomy. Historical patterns of isolation among populations play an important role in studying the variation in behavior, habitat, and other ecological interactions among different populations of a species or species group. Idotea balthica (Isopoda: Valvifera) is a common intertidal grazer of the North Atlantic which has been well-studied in Europe for intraspecific differentiation. Using DNA sequence data from a mitochondrial protein-coding gene (cytochrome c oxidase I), I studied the relationships among different populations of I. balthica in European and American coastal populations. It is apparent that there are at least three historically isolated populations of I. balthica on the North American coast, while the European Atlantic coast contains populations that are closely related to each other and to one of the North American populations. It appears that populations of I. balthica on the North American coast represent both recently arrived colonists from Europe as well as populations which have survived recent glacial maxima.