More than 900 individually marked island night lizards (Xantusia riversiana) were captured on San Nicolas Island, California, between 1984 and 2007 as part of an ongoing study to monitor the status of this threatened species. Our data suggest that at least a few lizards are probably more than 20 years old, and one lizard would be 31.5 years old if it grew at an average rate for the population. Ages of 20 and 30 years seem reasonable given the remarkably slow growth during capture intervals of more than a decade for five of the lizards which we estimated to be 20 or more years old. Like other lizards, island night lizard growth rates vary by size, with larger lizards growing more slowly. In general, growth rates were somewhat greater on San Nicolas Island (compared with Santa Barbara Island), and this increase was sustained through all of the intermediate size classes. The higher growth rate may account for the somewhat larger lizards present on San Nicolas Island, although we cannot discount the possibility that night lizards on San Nicolas are merely living longer. The high percentage of small lizards in the Eucalyptus habitat might seem to reflect a healthy population in that habitat, but the high proportion of small lizards appears to be caused by good reproduction in the 1900s and substantially poorer reproduction in subsequent years. The Eucalyptus habitat has dried quite a bit in recent years. Night lizards in the Haplopappus/Grassland habitat have shown an increase in the proportion of larger lizards since 2000. There has also been an increase in the proportion of large lizards in the Rock Cobble habitat at Redeye Beach. However, there are has been some change in habitat with more elephant seals occupying the same area just above the high tide as do the night lizards. Southern alligator lizards and side-blotched lizards are both non-native on San Nicolas Island. Neither lizard causes obvious harm to island night lizards, and management time and effort should be directed toward much more pressing problems, such as general habitat restoration, erosion control, and the removal of feral cats. The island night lizard (Xantusia riversiana) is endemic to three of the California Channel Islands: Nicolas, San Clemente, and Santa Barbara Islands. Due to its restricted range and apparently small population levels, both the U.S. Fish and Wildlife Service and the California Department of Fish and Game have listed the island night lizard as a threatened species. Our study was conducted on San Nicolas Island, which lies offshore 120 km southwest of Los Angeles, California. The island is managed by the U.S. Navy who refers to the island as Outlying Landing Field San Nicolas Island. The Navy maintains radar, telemetry, and communications equipment on San Nicolas Island to support its mission of testing and evaluating weapons systems. The Navy has dual requirements for ensuring military readiness and sustainability while complying with the Federal Endangered Species Act. A comprehensive understanding of the status and stability of the species on San Nicolas Island is essential for effective island management and may aid in the eventual delisting of the species. Previous work on the San Nicolas Island (Fellers and others, 1998) demonstrated that island night lizards were distributed over the eastern half of San Nicolas Island where there is suitable shrubby habitat. On the eastern half of the island, they occur primarily in or near cactus/sage scrub habitats on the north beach terrace, in scattered patches of scrub on the central mesa, and in boulder and cactus habitats on the southern escarpment of the island. Fellers and others (1998) evaluated data from 1984-85 and 1992-95 and estimated that there were 15,300 island night lizards present on San Nicolas Island. There are two non-native lizards on San Nicolas Island, the side-blotch lizard (Uta stansburiana) and the southern alligator lizard (Elgaria multicarinata). Both of the