The Vulnerable endemic Kloss's gibbon was surveyed throughout the Mentawai Islands, Sumatra, Indonesia, using a call-based method. Populations were estimated by extrapolating from local densities using recent assessments of remaining forest cover. About 20,000–25,000 Kloss's gibbons remain in nearly 3,000 km2 of forest (about 40% of the islands' total area). This number is markedly lower than a 1980 estimate of 36,000 on the northernmost island of Siberut. Conservation action to preserve the remaining populations should focus on strengthening the enforcement of regulations in Siberut National Park, formally protecting the Peleonan forest in north Siberut, and cooperating with a logging company in North and South Pagai to protect the populations in the company's self-established conservation and buffer areas.