The Black Hills are a mountain island surrounded by prairie on the eastern edge of the Rockies. The ant fauna of this area is known only from collections in the southern Black Hills and surrounding plains. To assess ant biodiversity and activity levels in the northern Black Hills, ants were collected monthly from April through September 2016 at two sites (1250 m ponderosa pine – common juniper forest and 2060 m aspen-pine forest). At each site, collections were made by opportunity collections and using 40 pitfall and 50 baited traps along perpendicular transect lines. We collected a total of 6180 ants. At the lower elevation, we identified nine genera consisting of 21 species in comparison to the higher elevation where we found only seven genera consisting of 13 species. Fourteen species and three genera were unique to the lower site while six species and one genus were unique to the higher site. Species accumulation graphs and richness estimates based on incidence data suggest more than 67% of the total number of species were collected at each of the sites. Diversity and evenness averages over the season are not significantly different between the two sites. Some of the species we collected are new records for South Dakota, with one well outside its known range.