Young horn sharks (Heterodontus francisci) were placed in an apparatus where they could choose between two chambers whose water temperatures differed by 10 °C. When the warmer chamber was at 26 °C, the sharks spent more time in the 26 °C water when the brain stem was cooled than when it was heated. With the brain stem kept constant at 12 °C, the sharks spent more time in the warmer chamber when it was at 26 °C than when it was at 31 °C. These findings demonstrate the importance of peripheral and rostral brain stem temperatures in the thermoregulatory behavior of a primitive member of the class Chondrichthyes.