A population of desert tortoises (Gopherus agassizii) at Yucca Mountain (Nevada, USA) was monitored during four sampling periods using enzyme-linked immunosorbent assays (ELISA) to determine the percentage of individuals that had been exposed to Mycoplasma agassizii, a causative agent of upper respiratory tract disease. Respiratory tract disease has been considered a significant factor in the decline of desert tortoise populations in the Mojave Desert (USA). Few differences between sexes in ELISA values or percentages testing positive were noted. From 15 to 23% of samples per period tested positive for exposure to the mycoplasma. However, we noted few clinical signs of upper respiratory tract disease. This is in contrast to an earlier study which reported a similar proportion of seropositive tortoises as well as a high percentage of tortoises with clinical signs. However, our results are consistent with that study's conclusion that seropositivity for M. agassizii was a poor predictor of the likelihood to exhibit clinical signs of upper respiratory tract disease. Earlier reported epizootics of mycoplasma-associated respiratory disease occurred mainly during times of drought. Our samples were collected during a period of average to above-average rainfall, suggesting that manifestation of clinical signs of the disease may depend upon the physiological condition of tortoises which, in turn, is related to environmental conditions.