We conducted spotlight counts along a 41-km transect to estimate population density and habitat use of mule deer (Odocoileus hemionus), an introduced mammalian herbivore on Santa Catalina Island, California. Mean population density, calculated using distance-sampling procedures, was 5.41 deer/km² (95% CI: 4.40, 6.65) during the 11 months of our study. Deer used island chaparral, human-developed areas, and to a lesser degree, riparian vegetation more than expected, and bare areas, grasslands, and woodlands less than expected, based on the availability of these habitats in our study area.