At the extreme southwestern part of its range in southeastern Arizona, annual surface activity of the Desert Box Turtle (Terrapene ornata luteola) is limited primarily to the three-month summer monsoon. Turtles emerging at the beginning of the monsoon are severely dehydrated. We examined the effects of dehydration on the critical thermal maximum (CTMax) of T. o. luteola. Compared to normally hydrated turtles, the CTMax of dehydrated turtles was depressed about two degrees from 41.0–39.0°C.