Five hundred ten measurements of swimming speeds of skipjack tuna (Euthynnus pelamis) from four schools and 33 of yellowfin tuna (Thunnus albacares) from one school were made from underwater motion-picture records obtained at sea. The relation of swimming speed to rate of tail beats is described for yellowfin averaging 51.9 cm long and for skipjack averaging 57.2 cm long. At similar rates of tail beats, yellowfin swam faster than skipjack when speed was measured in body lengths per second. Variations in the mean swimming speed of the schools of feeding skipjack were unrelated to body length. The difference between the swimming speeds of tunas observed during this study, which ranged from 0.5 to 14.4 body lengths per second (0.3 to 6.9 m per second), and higher speeds previously reported for tunas is discussed.