Activities and habitat use of lesser mouse-deer (Tragulus javanicus), a common but poorly studied ruminant native to lowland tropical forests of Southeast Asia, were investigated by full-day radiotracking and direct visual observations in the Kabili-Sepilok Forest Reserve, Sabah, Borneo. The mouse-deer was thought to be nocturnal, but diurnal distance moved per hour and behaviors observed directly in wild mouse-deer indicated that they were mainly active during the day and rested at night. Active individuals foraging or moving from shelter to shelter were mainly observed during the day, and inactive individuals resting on open forest floor were mainly observed during the night. Captive mouse-deer observed in a farm enclosure (1.5 ha in size) also showed activities similar to those observed in the forest. Radiotracking for 24 h revealed that mouse-deer significantly preferred crown-gap areas with dense undergrowth of creeping bamboo (Dinochloa) during the day but that they tended to move to the higher and drier ridge areas at night. These results indicated that mouse-deer used crown-gap areas as foraging sites and ridge areas as resting sites. Our results strongly suggest that mouse-deer use food resources in gap areas in tropical forests.