West Indian rock iguanas (genus Cyclura: Iguanidae) are among the most endangered lizards in the world, and many species will need to occupy human-modified and -occupied habitats to escape extinction. The Grand Cayman Blue Iguana, Cyclura lewisi, is critically endangered with fewer than 25 wild iguanas remaining. To aid the conservation of this and other iguanas, we investigated the spatial ecology of a captive-bred, released population of C. lewisi occupying a botanic park on Grand Cayman. Iguanas were monitored using transect walks and radio telemetry during the summer and fall of 2002. Males used larger areas and had greater movement distances than females during tracking periods in the summer but not in the fall. Overall home ranges for both seasons combined were larger in males than in females. Some home-range estimates were greater than any previously reported in Cyclura. Several iguanas, especially males during the breeding season, used areas outside the park where they are vulnerable to increased predation, death by vehicle, and hunting or collection by humans. This, combined with the large average home-range sizes for this species, indicate that future reserves for C. lewisi should be large and surrounded by buffer zones or fences.