The relict leopard frog (Rana onca) was once thought to be extinct, but has recently been shown to comprise a valid taxon with extant populations. We delineate the minimum historical range of the species based on records from 24 localities, report the extinction of 2 of 7 populations extant in the 1990s, and estimate total population size. The 5 remaining populations occurred in 2 areas: near the Overton Arm of Lake Mead and in Black Canyon along the Colorado River below Lake Mead, Nevada. These 2 areas are only 3.6 and 5.1 km long, respectively. The 5 extant populations inhabited spring systems with largely unaltered hydrology and no introduced American bullfrogs (Rana catesbeiana) or game fishes. In a mark-recapture study conducted in the Overton Arm area, the estimated number of adult frogs averaged 36 over 555 m of stream habitat, and estimated annual survivorship of adults averaged 0.27. A single mark-recapture estimate for the size of the largest population within Black Canyon, at a site approximately 450 m in length, was 637 adult frogs. An estimate for the total number of frogs at all sites, based on mark-recapture data, visual encounter surveys, and extent of habitat, was approximately 1,100 adults (range 693–1,833). The 2 recent population extinctions occurred concomitantly with encroachment of emergent vegetation into pools. We speculate that this occurred as a result of natural processes in one case, and anthropogenic processes in the other.