ABSTRACT:  The desert tortoise, Gopherus agassizii, and its habitats in the western Mojave Desert and elsewhere are negatively affected by off-road vehicles (ORVs).  Data from a study conducted at the Desert Tortoise Research Natural Area during 1992 provide insights into why ORVs are likely to affect tortoises.  To determine habitat use and food preferences, 18 large immature and adult tortoises were observed.  The study site contained four subhabitats or strata:  washes (comprising 7.9% of the area), washlets (2.4%), hills (42.3%), and flats (47.4%).  The tortoises used the four habitat strata differentially, spending significantly more time (92%) in washes, washlets, and hills throughout spring than in the flats (8%).  They were observed to take bites from 2,423 individual plants of at least 43 plant species (37 annual, 6 perennial).  They showed preferences for native plants (95.3% of bites) compared to non-native plants.  Some of the ten most-preferred food plants were uncommon to rare in the environment.  Three of the ten most-preferred food plants occurred largely in the wash strata, and an additional four species were found only in hill strata.  Users of recreational vehicles also prefer washes and hills in this region, where they are more likely to encounter tortoises, increasing the possibility of direct mortality, and where they are more likely to have a greater impact upon preferred forage and habitats.