The Kloss Gibbon (Hylobates (Hylobates) klossii), also known as the Bilou, is a primate in the Hylobatidae or gibbon family. It resembles the Siamang with its black fur, but is somewhat smaller. Kloss Gibbons reach a size 44 to 63 cm and weigh at most 6 kg. As is the case for all gibbons, they have long arms and no tail.

Kloss Gibbons exclusively live on the Mentawai Islands that lie to the west of Sumatra. They are diurnal inhabitants of the rain forest, that hang in the trees from their long arms and rarely come to the ground. Like all species of gibbons they live together in pairs that stake out a territory from approximately 20 to 30 hectars of size. This area is defended vehemently against other gibbons. Their diet consists mainly of fruits, occasionally also eating different plant parts, bird eggs and small vertebrates.

The singing of the Kloss Gibbons is considered the most beautiful of all the gibbons' songs. Males and females will sing at considerable lenght either singly or in duets, particularly during the morning and evening. The singing of the gibbons serves to warn off other animals from their territory, and possibly to strengthen the family bonds.

The reproductive cycle of the Kloss Gibbons is similar to that of other gibbons. Every two to three years the female may give birth to a single young (with a gestation period of seven months). The young is weaned in the middle of its second year, and is fully mature in about seven years. Their life expectancy is about 25 years.