1. The behavior of a breeding colony of larger Malayan mouse deer was observed for seven months. 2. Mouse deer produce a noise by stamping with one or both hind feet when slightly alarmed. Other individuals may or may not stamp in response. 3. Both males and females mark objects with the inter-mandibular gland. Males mark much more frequently than females. 4. Males often lick the urine of females; less frequently, females lick the urine of males. Mouse deer do not "flehmen" in response to urine. 5. Males court both receptive and unreceptive females. Courting males mark the female on the back or rump with the inter-mandibular gland and emit a series of squeaks. The behavior of both unreceptive and receptive females and copulation are described. 6. Females have a post-partum estrus and return to estrus at approximately 14 day intervals unless they become pregnant. 7. Mothers spend little time with infants. Mothers emit a vocalization which sounds like the squeak of courting males. Both mothers and infants emit a higher pitched vocalization. If her infant emitted this vocalization the mother answered and approached and stood by it. The mother nurses in a standing position and raises the hind leg on the side towards the infant. 8. Intense agonistic behavior was seen only when strange individuals were introduced into established groups. Males fight by facing each other and biting each other on the ears, neck, and shoulders with their large upper canines. Fighting males usually hold the tail in a vertical position, exposing the white ventral surface, and may emit loud growls. If one male flees, the other pursues and attempts to bite him on the neck and body. 9. Mouse deer are morphologically primitive and many of their motor patterns are also thought to be primitive. 10. The behavior of all four living tragulid species appears to be quite similar, both with respect to motor patterns and social behavior.