Repeated surveys of the butterflies of Monhegan Island, ME, from 1998 to 2018 confirmed the presence of at least 40 species. This total represents more than 42% of the state's recorded species—a high proportion given the island's small size and isolation. The rate of discovery indicates that, based on observations accumulated over time, about 50 species could be observed on Monhegan, including large migratory taxa as well as a variety of habitat-limited sedentary residents. The absence of several mainland species can be attributed to limited habitat and lack of some plant species. Records show changes in the butterfly fauna over recent decades; further changes may be expected because of climate warming, habitat modification, and stochastic events.