Field studies were made of body temperature and flight activity of the alfalfa butterfly, Colitis philodice eurytheme Boisduval, in relation to solar and other radiation, light, air temperature, and moisture. Some of the response factors were also studied under controlled laboratory conditions. Solar radiation appears to be the most important agent affecting body temperature, but its effect is in turn modified by air temperature and the cooling of the insect through evaporation. Under sufficiently high radiation, and when air temperature is not too low, Colias increases its body temperature by orienting perpendicular to the sun. When body temperature nears the critical high level, Colitis appears able to regulate it by decreased flight, rest in shade or on moist soil or algal scum, or by taking water while at rest, thus permitting cooling without desiccation. When air temperature and radiation approach limiting levels, however, desiccation can limit flight even if body temperature is below the upper critical level, especially where there is no source of water or nectar. On the other hand, peak solar radiation may not limit flight if low air temperature and evaporation keep the body temperature within the favorable range for activity. Low light intensity is the only factor of diose studied that, alone, caused cessation of flight when all other factors were favorable.