The western harvest mouse, Reithrodontomys megalotis, is widely distributed, occurring in much of the western and central United States, as well as in much of Mexico and parts of southwestern Canada. In Utah, the species can be found state-wide, in heavily vegetated areas near water. Preferred habitats include meadows, fields, weedy areas, and grasslands.
The western harvest mouse feeds primarily on weed and grass seeds, but other plant material is also consumed. Nests are constructed above ground, often under thick vegetation or in shrubs or vines. The species breeds year-round; gestation lasts about three to four weeks, and the average litter contains four young. Western harvest mice become sexually mature in only two to four months.
Western harvest mice are active throughout the year, but are primarily nocturnal.