The present work examines the thermal preference of adult Rhodnius prolixus along a temperature gradient. The mean preferred temperature differed slightly between sexes: 25.0 degrees C for males versus 25.4 degrees C for females. This preference was not constant, but varied daily by about 0.2 degrees C for both sexes, and reached its highest value at the onset of the dark phase and was lowest during the light phase. A change in the preferred temperature with the level of starvation was also observed (about 1 degrees C lower after 20 days of starvation). Changes in environmental temperature strongly affected the rate of weight loss for both sexes. When insects were maintained for 20 days in a chamber at 32 degrees C, they lost significantly more weight than when kept at 24 degrees C; both water loss and nutrient conversion processes are involved. This increase in weight loss rate with increasing temperature would cause a higher biting rate and consequently higher probability of Chagas' disease transmission. Females oviposit across a range of temperatures from 22 to 33 degrees C with a peak at 25-26 degrees C. These results are compared with patterns of thermopreference in other species of triatomine, as related to differences in their distribution and tolerance to starvation.