Limacina helicina (1-3 mm) lives in the environment that straddles both inertial and viscous regimes. In this intermediate Reynolds range (10(0)-10(3)), an oscillating appendage may use either drag-based or lift-based locomotion. The swimming motion of L. helicina was investigated to determine its mechanics and whether features of rowing or flying gaits were present. Mean speeds, stroke frequencies, and general paths were revealed from the trajectories of free-swimming individuals. High-speed videography of tethered animals enabled a detailed analysis of stroke parameters involved in L. helicina swimming. During swimming episodes, L. helicina ascend along a sawtooth trajectory in mostly linear and sometimes helical paths. Mean speeds varied from 13 to 44 mm/s for straight ascents and slightly more for helical paths. During swimming, the stroke cycle caused oscillations in body orientation, whereas sinking is characterized by smooth straight descents. Sinking speeds of 5-45 mm s(-1) were observed. Wing-beat frequencies decreased with body size from 4.5 to 9.4 Hz. The wing stroke is a complex, three-dimensional motion that does not perfectly correspond to theoretical concepts of drag-based or lift-based propulsion. Instead, the repertoire of movements indicates that elements of both rowing and flying are incorporated in the swimming of L. helicina with the added element of rotation. Size-dependent differences in stroke mechanics are described. Of particular note is evidence that a clap-and-fling mechanism is applied during the stroke cycle.