A common garden experiment was used to test for genetic differentiation between populations of the widespread Mimulus ringens var. ringens and the very locally distributed var. colpophilus, in and around Merrymeeting Bay in Maine. Measurements in the field documented a statistical difference for leaf length and calyx lobe length between populations found in non-tidal versus tidal waters, although most populations surveyed fell within values originally established for var. ringens. Field-collected seeds grown in the greenhouse showed differentiation between varieties for flowering date and for three of four of the morphological characters measured. A second generation grown in the greenhouse showed differentiation between varieties for the same three morphological characters, but not for flowering date.