Subcutaneous inoculation of 34 Texas tortoises (Gopherus berlandieri) with either of two strains of Western Equine Encephalitis (WEE) virus resulted in prolonged viremia of up to 105 days' duration. Peak viremia titers exceeded 106 suckling mouse intracranial lethal doses per milliliter. The length of the previremic period, the maximum viremia level attained, and the duration of viremia were markedly affected by environmental temperature. Higher temperatures (30° C) shortened the previremic period and the duration of viremia and elevated the maximum viremia level. Lower temperatures had the opposite effect. Mechanisms are suggested whereby Texas tortoises could serve as an overwintering reservoir for WEE virus at any temperature. Neutralizing antibody was detectable following viremia in only 11 of 16 (69%) of surviving tortoises.