SOUND production is one of the most energetically costly activities in animals(1), Minimizing contraction costs is one means of achieving the high activation rates necessary for sound production (20-550 Hz) (refs 1-3) without exceeding energy supplies. Rattlesnakes produce a sustained, high-frequency warning sound by extremely rapid contraction of their tailshaker muscles (20-90 Hz) (refs 4, 5), The ATP cost per twitch is only 0.015 pmol ATP per g muscle per twitch during rattling, as measured by in vivo magnetic resonance. The reduced volume density of myofibre (32%) in tailshaker muscle is consistent with contraction cost being minimized (crossbridge cycling), in contrast to the con tractile costs of vertebrate locomotory and asynchronous insect flight muscle, Thus tailshaker muscle is an example of sound-producing muscle designed for ’high frequency, minimal cost’. The high rates of rattling are achieved by minimizing contractile use of ATP, which reduces the cost per twitch to among the lowest found for striated muscle.