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We studied the effects of various shelter and prey odor combinations on selection of microhabitat characters by the Desert Nightsnake, Hypsiglena chlorophaea, a dipsadine snake. We also examined the activity patterns of these snakes over a 23-h period. Three prey odors were tested, based on field work documenting natural prey In Its diet: lizard, snake, mouse (plus water as control). In the first experiment, each odor was tested separately In various shelter and odor combinations. We found that snakes preferred shelter to no shelter quadrants, and most often selected a quadrant If It also had prey odor In the form of lizard or snake scent. However, snakes avoided all quadrants containing mouse (adult) odor. In the second experiment, all three odors plus water were presented simultaneously. We found that snakes showed a preference for lizard odor over the others, but again showed an aversion to mouse odor, even compared to water. The circadian rhythms In both experiments showed generally the same pattern, namely an Initial peak In activity, failing off as they entered shelters, but then again increasing even more promlinently from lights off until about midnight. Thereafter, activity tapered off so that several hours before lights on in the morning, snakes had generally taken up residence In a shelter. Prey preference correlates with field studies of dietary frequency of lizards, while activity exhibits strong endogenous nocturnal movement patterns.