Abstract: An assistive aquatic propulsion device affixable to a wearer's body to increase speed and mobility of bodily movement during surface and underwater swimming. The device includes a flexural fin member which in one embodiment of the invention is joined to a transversely extending spar member joined to a longitudinally extending yoke member, with the yoke member being releasably secured to the legs of a wearer, and with the spar member being arranged perpendicularly to the longitudinally extending yoke member. In a specific embodiment, the device includes a biomimetic tailfin simulative of tailfin structure of a whale, porpoise, manatee or other marine animal.