SUMMARY1. Scorpions Buthus occitanus and Androctonus australis were placed in a temperature gradient where they could select their thermopreferendum. Intrathoracic temperature was recorded continuously.2. Both species selected 24*8 + 1 0 0C as their preferred body temperature. No nycthemeral cycle of preference was observed in the experimental conditions. Saline injection did not modify this thermopreferendum.3. Prostaglandin E1 (PGE1) produced a fever. Duration and magnitude of fevers were related to PGE1 doses in a bell-shaped curve. The longest and highest fevers were obtained with 4 jg . g-1 PGE1.4. These results show that there is an ability to produce fever and, thus, indirectly, that there is a set point in body temperature regulation in arthropods, among the oldest known terrestrial animals.