We investigated the circadian activity rhythm in the little-studied Wagner's gerbil (Gerbillus dasyurus) from the Raydah Protected Area, southwestern Saudi Arabia. In order to assess whether these animals possess an endogenous rhythm of locomotor activity that entrains to the light:dark cycle, they were subjected to three distinct light cycles: an LD cycle (12 h light/12 h dark), a DD cycle (constant darkness) and a DL light cycle (an inverse of the LD light cycle). All eight individuals studied exhibited entrainment of their activity to the light cycles. Under LD, the total percentage of activity during the dark phase was 93.7 ± 1.8%. Activity was distributed throughout the night (mean peak activity 22:46 ± 0:14). All eight animals expressed distinct endogenous free-running rhythms of locomotor activity (mean í = 23:55 ± 0:36). During constant darkness, animals still displayed more activity during the subjective night (75.6 ± 0.4%). Under the DL light cycle, the total percentage of activity was 92.7 ± 1.8% during the dark phase. In conclusion, the daily locomotor activity rhythm of Wagner's gerbil is strongly entrained by the light:dark cycle with the most activity concentrated during the night and consequently this desert-dwelling mammal may thus be considered truly nocturnal.