In 1995, the Sino-Belgian dinosaur expedition discovered a rich bonebed in the Iren Dabasu Formation (Late Cretaceous) of Erenhot (Inner Mongolia PR China). This bonebed comprised the scattered skeletons of at least four specimens belonging to the primitive hadrosaur Bactrosaurus johnsoni Gilmore, 1933. The study of the new material allows an accurate osteological reconstruction of this species, eg definitely showing that it was a flat-headed hadrosaur. The superfamily Hadrosauroidea is diagnosed as a monophyletic groups characterized by 11 synapomorphies. Bactrosaurus johnsoni is the most basal taxon of the Hadrosauroidea, as it lacks numerous apomorphies observed in the other well known taxa within this superfamily. The basal position of Bactrosaurus johnsoni in hadrosaur phylogeny speaks for an early Late Cretaceous age for the Iren Dabasu Formation.