The Tibetan Babax is a high-altitude babbler endemic to the NE Tibetan plateau. This is the first report of the species nesting behaviour: the study was based on information gathered during 2005 and 2006 in the upper Mekong River basin (32°N, 96°E, 3700–4300 m.a.s.l.), S Qinghai. The birds nested low (< 2 m) in conifers and laid eggs from early May to late July. The clutch size was 3–4 eggs. Nestlings fledged at 13–14 days, when they reached 52% of the adult female weight. In all four nesting attempts (three nests and one post-fledging social unit), more than two birds (3-6) were observed feeding the young or guarding the nest against conspecific intruders or potential predators. These results indicate that all three member species of the genus Babax (the two others are the high-altitude Giant Babax and the lowland Chinese Babax) display a cooperative breeding system. This provides a good example of phylogeny as a component responsible for the evolution of cooperative breeding behaviour.