The Nerodia erythrogaster neglecta (Copper-bellied Watersnake) population north of 40 degrees north latitude is categorized as threatened under the US Endangered Species Act. Previous efforts to use mark-recapture to estimate population size have been unsuccessful because there were so few initial captures. We therefore evaluated the status of the largest presumed subpopulation of this population by using strip transects to provide a conservative (maximum) estimate of density and population size of adults. Our results indicate that the largest presumed adult Copper-bellied Watersnake metapopulation north of 40 degrees north latitude is extremely small, with a total size of 94 ± 22 adults, and is characterized by a population density of 1.17 ± 0.27 adult snakes/ha (95 % CI = 0.73–1.87 snake/ha). This Copper-bellied Watersnake population appears to be in danger of extirpation within the foreseeable future. Recovery will require protecting existing wetland complexes and restoring wetlands and the surrounding upland habitats.