Sky islands are often areas of endemism and high species diversity around the world. In central North America, the Ozark and Ouachita Highlands contain numerous modestly high elevational islands that are slightly cooler and wetter than surrounding lowlands and these areas are known to contain many unique species. We studied parasitic Hymenoptera biodiversity on three sky islands in Arkansas, both in canopy and understory strata of mature post oak forests. We found that morphospecies diversity was high, and community structure varied both within (canopy versus understory) and between sky islands. Each mountaintop showed a high level of distinctiveness with few morphospecies found in multiple locations. The high intersky island variation indicates that individual sky islands in the region harbor unique Hymenoptera assemblages, a pattern that likely extends to the broader insect community. Our study suggests that these highland areas are important regions of North American biodiversity and that they should be evaluated individually for conservation efforts in order to preserve their distinctive community structure.