The underparts of the body are white in both species. O. arenicola superficially resembles O. torridus but averages smaller in size and differs in karyotype. The genus is distinguished by the stocky body and fairly short, clublike tail. In O. leucogaster the tail is usually less than half the length of the head and body, but in O. torridus it is usually longer than half the length of the head and body. Female Onychomys have six mammae.
Grasshopper mice occur in shortgrass prairies and desert scrub.
Both species are largely nocturnal and are active throughout the year. They live in practically any shelter they can find at ground level. They are good climbers but apparently do not climb regularly. The nest may be constructed in a burrow taken over from some other rodent. O. leucogaster reportedly excavates a U -shaped burrow with an average length of 48 cm and an average depth beneath the surface of 14 cm.