Psittacosaur materials were collected from seven localities in the course of Dinosaur Project fieldwork in Inner Mongolia. Two new species of Psittacosaurus are recognized, bringing the number of well-defined species to six. One, based on a nearly complete skeleton, resembles Psittacosaurus mongoliensis, but is a smaller animal with a relatively larger head and longer tail. The other species possesses jugal horns, but exhibits a different combination of characters than Psittacosaurus sinensis and Psittacosaurus xinjiangensis. A larger matrix of taxa and characters than is presently available will be necessary to resolve adequately phylogenetic relationships within the genus. Psittacosaurs were small dinosaurs, the skeletons of which are preserved in unusual completeness in proximity to aeolian environments of deposition. Their small brain size relative to that of modern mammals of similar body size implies a relatively restricted behavioral repertoire.