The authors examine the impact of environmental and political variables on two measures of species imperilment across 49 U.S. states: the fraction of all species in a state identified by NatureServe as being “at-risk” of extinction, and the fraction of species in a state listed under the U.S. Fish & Wildlife Service's (FWS) Endangered Species Act (ESA). A highly significant determinant of both measures is the fraction of species endemic to the state. Population growth increases the at-risk measure, but not the ESA listings. There is a significant concern for plant imperilment by NatureServe, but not by the FWS.