Jet-propulsion in dragonfly larvae is achieved by the rapid ejection of water from a specialised rectal chamber via the anus, at a frequency of up to 2.2 cycles/s. Movement, forward thrust and muscular activity have been recorded in restrained and free-swimming larvae. Forward thrusts of up to 1.5 g wt result from the expiratory phases of cycles lasting 0.1 to 0.4 s. Swimming velocities are in the order of 10 cm/s. The following muscles are shown to be active during expiratory phases of jetting: anterior, posterior and respiratory dorso-ventrals; primary and secondary longitudinal tergals; lateral primary longitudinal sternopleural; dorso-ventral oblique; ventral adductors of the anal appendages. The sub-intestinal muscle is active during the inspiratory phases of jetting. Activity recorded is compared with that found during normal ventilation. The larval jet-propulsive mechanism is compared with that of certain cephalopods and found to be very effective for the larva's relatively small size.