The Mecyclothorax (Sharp) fauna of isolated forest habitats in and near Polipoli State Recreation Area, Kula Forest Reserve, Maui Island, Hawaii comprises eight species, five of which are newly described: Mecyclothorax aeneipennis, new species; M. consobrinus, new species; M. cordithorax, new species; M. giffardi, new species; and M. superstriatus, new species. An identification key to Mecyclothorax species of Polipoli is presented along with diagnoses, distributional maps and illustrations. Lectotypes are designated for M. laetus (Blackburn), M. ovipennis Sharp, and M. perstriatus Sharp. Four species—M. aeneipennis, M. consobrinus, M. giffardi and M. superstriatus—are precinctive to Polipoli, and constitute respective adelphotaxa to four allopatric species distributed from the Waikamoi region thence eastward along the windward face of Haleakala Volcano. The Polipoli endemics define the forest regions of Kula Forest Reserve as an area of endemism distinct from the windward forest to the northeast. Incorporation of adelphotaxic distributional information into the delimitation of these areas of endemism provides a cladistic foundation that is lacking from most other previously proposed definitions, though it harkens back to Wallace's (1855) “Law which has regulated the introduction of new species.” Native Mecyclothorax beetles occur in both native and non-native forest microhabitats at Polipoli, illustrating the ability of these geographically restricted island endemic taxa to colonize and persist in novel situations populated by alien plants.