Near the Straits of Mackinac, the Limenitis arthemis population on Michigan’s Upper Peninsula consists mostly of the disruptively banded L. a. arthemis, while the population on its Lower Peninsula consists mostly of the unbanded, mimetic L. a. astyanax and arthemis-astyanax intergrades. Except at the straits, the Upper and Lower peninsulas are broadly separated by lakes Michigan and Huron. On the Lower Peninsula, arthemis-like forms are most common on the shore close to the Upper Peninsula but are much less frequent only 20 km south, probably because of the northward flow of astyanax genes. Neither population is in Hardy-Weinberg equilibrium because of a deficiency of "heterozygotes," possibly because of croos-lake emigration at the straits.