Upper Respiratory Tract Disease (URTD), caused by the pathogens Mycoplasma agassizii and M. testudineum, has been documented in the desert tortoise (Gopherus agassizii). Although URTD was identified as the putative agent that led to federal listing of the Mojave population of the desert tortoise, little is known about this disease in the Sonoran population of the desert tortoise. The purpose of this study was to determine: (1) the prevalence of URTD across an urban gradient in Greater Tucson, Arizona, (2) the relationship between URTD and captive and free-ranging tortoises in Mohave, Maricopa, and Pima counties in Arizona, and (3) the effects of URTD on desert tortoise home range size and winter temperature selection. To determine the prevalence of M. agassizii, I used enzyme-linked immunosorbent assay (ELISA) to detect anti-M. agassizii antibodies in plasma samples, indicating previous exposure, and polymerase chain reaction (PCR) to detect M. agassizii DNA in nasal flush samples, indicating a current infection. I found that tortoises from suburban sites are 2.3 times more likely to test seropositive for antibodies to M. agassizii than tortoises from other sites in the Greater Tucson area. When I compared the seropositivity between captive and free-ranging desert tortoises from high-visitor-use site tortoises in Mohave, Maricopa, and Pima counties, I found that captive desert tortoises are 1.8 times more likely to test ELISA-positive than free-ranging desert tortoises, and desert tortoises in Pima County are 5.4 times more likely to test positive for anti- M. agassizii antibodies. When examining the effects of URTD on behavior, I found no significant difference between seropositive and seronegative tortoises for home range size, and the two clinically ill desert tortoises exhibited daily activity during winter that resulted in increased environmental temperatures. Although additional study is needed to better understand the dynamics of M. agassizii in desert tortoise populations, our results are consistent with monitoring results that suggest that M. agassizii is widespread among tortoises in the Sonoran Desert and provides additional indirect evidence that captive tortoises are likely an important reservoir of URTD and may pass this disease onto free-ranging tortoises.