We studied the population biology of the Spiny-Tailed Iguana (Ctenosaura bakeri) endemic to the Honduran island of Utila and previously assumed to be close to extinction. Here, for the first time, we present a comprehensive mapping of its overall distribution and nesting area. The total range of C. bakeri comprised about 1,091 ha of mangrove swamp. Nesting sites were restricted to 109 ha of sandy coastal territory. We used two closed capture-recapture models to estimate adult population densities at 35–78 and 72–114 adults/ha within three mangrove areas, respectively. Population densities were largely related to the presence of suitable retreats. Population estimates for the entire mangrove habitat based on these mean adult densities ranged from 57,823–93,826 individuals, far higher than earlier reports have suggested. Also, no evidence was found for an unbalanced demographic structure. About 27% of all iguanas captured were juveniles (<150 mm snout–vent length). Notably, juveniles were difficult to detect and capture, and additional sightings of 0.8–2.3 juveniles/day indicated even higher abundances than given by captures. The sex ratio of males to females (1 ∶ 1.2) was consistent with reports of adult social groups in other large iguanids. Threats to C. bakeri included hunting and habitat pollution but mainly the loss of habitat and nesting sites caused by unrestricted property development. Without proper habitat management, we anticipate significant declines of the C. bakeri population within the next 20 yr.