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Microhabitat use by deer mice (Peromyscus maniculatus) was tested under five different simulated moonlight intensities, using an indoor habitat chamber with shrub covered areas and open areas, and food sources. Tests were run in the presence and absence of a potential competitor, Ord’s kangaroo rat (Dipodomys ordii). From tracks in the sand, we determined that, when alone, deer mice decreased total activity as moonlight increased and were more active in areas with cover than in open areas. However, deer mice did not vary proportions of activity allocated to open areas (ca. 20%) and cover/edge areas (ca. 80%) with variations in moonlight. Deer mice consumed more seeds in areas with cover but also did not vary the proportions of seeds eaten in the open (ca. 43%) or cover (ca. 57%) with variations in moonlight. Using infrared video filming, we determined that deer mice increased use of cover to nearly 100% in the presence of Ord’s kangaroo rats. Aggression (active chasing and locking fights) by kangaroo rats towards deer mice caused this shift. Our experiments substantiate field observations of variable microhabitat use by deer mice in areas with and without kangaroo rats and identify behavioral interactions involved.