To determine their capacity for thermoregulation, yellowfin tuna, Thunnus albacares, were subjected to a series of 12-h periods at $T_{a}'s$ of 20, 25, and 30 C. Muscle temperature, measured with an ultrasonic transmitter attached to the fish, and swim speed were simultaneously monitored. No relationship was found between speed and muscle temperature, although metabolic heat production is inexorably linked to the former. Because both direct and inverse muscle temperature/heat production relationships were observed, and because physical (as opposed to physiological) explanations for our data can be discounted, we hypothesize yellowfin tuna are capable of some type of central nervous system (CNS)-mediated physiological thermoregulation.