Interregional trade flows of Canada (provinces) and the United States (states), the world's largest bilateral trading partnership, are investigated using a measure of trade specialization similar to the location quotient and using provinces and states as the spatial units of analysis. Past research investigating the clusters in the geography of international trade flows have focused on national units of analysis, despite the fact that subnational units display geographical patterns of trade distinct from their respective nations. Regional trading relationships have evolved since the establishment of free trade between Canada and the United States as the result of decreased costs in accessing foreign markets. These new trading relationships now incorporate more U.S. states and fewer Canadian provinces in most cases.