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Most studies of deer activity occur during diurnal hours; nocturnal activity is rarely addressed. For desert mule deer (Odocoileus hemionus crooki), diurnal studies alone may not represent normal activity rhythms resulting from heat stress at certain times of the year. To understand diel activity patterns, nocturnal activity should be examined to see if and how diurnal and nocturnal activities differ. Thus, we quantified nocturnal activity of 5 radio-collared female desert mule deer in the Belmont and Big Horn mountains, Arizona in 1990. We determined their movements and estimated activity with observations and a digital processor. We compared areas used by deer at night with areas used during the day. Nocturnal activity was greater in warm seasons (spring, summer) than in cool seasons (autumn, winter) (P = 0.020). Deer moved an average of 778 +/- 85 (SE) m (n = 84) during a single nocturnal observation period. Most areas used by deer at night (88%) overlapped with daytime area use. Use of habitat was consistent between night and day in six of 8 vegetation associations. Use of disturbed sites increased at night (P < 0.01). Nocturnal use of habitats by female desert mule deer was representative of diurnal use of habitats and suggested that daytime locations sufficiently represented overall patterns of habitat use, except when unduly influenced by human disturbance.