Observations of Hylobates klossii were made during a one-week expedition to the offshore Mentawai Islands in western Sumatra. The 4 groups observed appeared to be monogamous families. Locomotive behavior of H. klossii seemed intermediate between that of H. syndactylus and H. lar. Animals were seen eating figs and berries. Protracted calling sessions were heard in the morning and evening; these may be suppressed by rain. The indigenous Mentawai people selectively log forests around their villages and occasionally hunt H. klossii for food with bow and arrow; these old established practices may reduce local population levels around villages, but probably have little effect upon the total species population. Some animals are captured for the pet and export trades, but the most serious threat to H. klossii is commercial logging by foreign companies which may be imminent on the Mentawai Islands.