Micarea micrococca and M. prasina are two morphologically similar crustose lichens that have been reported from North America. We present the results of the first large scale study of these species, and the first study of them outside of Europe. We examined over 200 specimens referable to these species from eastern North America to determine whether differences in morphology, ecology, or geographic distribution correlated with the differences in chemistry that distinguish the two species. Differences in morphology and ecology were not detected, but geographic distribution was found to be strongly correlated with chemistry. Nonetheless both species are entirely sympatric in the study area. Though overlapping, goniocyst size also appears to correlate with chemistry, a character that has not previously been explored in studies of this group. Treatments for both species are presented, including illustrations and distribution maps.