To understand the habitat components that contribute to the presence of populations of a rare butterfly, we examined the abundance of critical plant-components of old fields that support some of the last remaining Eastern Speyeria idalia (Regal Fritillary Butterfly) subpopulations at Fort Indiantown Gap (FTIG), a National Guard training facility in south-central Pennsylvania. We compared densities of larval-host plants (Viola spp. [violets]), adult-nectar plants (Asclepias spp. [native milkweeds] and Cirsium spp. [thistles]), and native, tussock-forming, warm-season bunch grasses that provide protective resting and pupation sites in fields occupied by the butterfly and in nearby fields that were unoccupied. We found no significant difference in violet density among sites. Fields with Regal Fritillary Butterfly populations had significantly more nectar-plant flowering structures and greater bunch-grass percent cover. Grassland habitat occupied by Regal Fritillaries was characterized by a violet density of at least 1.55 plants/m² and particular varieties of flowering nectar-plants available throughout the June–September flight period. Bunch grasses were also important to persistence of Regal Fritillaries; occupied sites had 20–45% bunch-grass cover and tussock formation. Understanding the habitat needs of this rare butterfly in Pennsylvania is vital to its restoration and reintroductions of the eastern form in the mid-Atlantic and northeastern US.