We describe aspects of the life history of Erynnis propertius (Scudder and Burgess) (Lepidoptera: Hesperiidae) by examining several populations over multiple years. We focused on peripheral populations of this species because they are isolated, are threatened by habitat loss, and may play an important role in driving poleward range expansion under increasing regional temperature. Our findings show that the annual larval growth rate does not vary directly with temperature, adult flight phenology and the timing of key resources respond to average daytime temperatures in spring, and population-density patterns among sites are robust over years across a broad region near the species' northern range limit. In addition, we provide descriptions of all larval instars for this species. This fundamental information about the biology, timing, and abundance of this species will facilitate further experimental study and improved assessment of its conservation status.