The rhizostome scyphomedusa Stomolophus meleagris swims continuously at speeds up to 15 cm∙s−1. Mean velocities increased as a power function of wet weight up to 70 g but were mostly constant thereafter. Bell pulsations ranged from 1.7 to 3.6 Hz. Reynolds numbers equalled 900 – 13 000. During activity, medusae consumed 0.05 mL O2∙h−1∙g WW−1 (1.2 mL O2∙h−1∙g DW−1), at 30 °C. Rates for inactive medusae were 50% less. The estimated cost of transport ranged from 2 J∙kg−1∙m−1 at 5 g to 1 J∙kg−1∙m−1 at 1 kg. These rates are comparable to those of fishes and about 1/50th that of planktonic crustaceans. These results were unexpected in light of the typical inefficiency (power output/power input) of jet swimming. However, S. meleagris has a very low respiration rate relative to crustaceans and fish, which probably compensated for low swimming efficiency.