The flapping wings of flying birds and bats generate complex, unsteady air movements. These consist of well-defined vortex structures which are a necessary result of the aerofoil action of the wings, and which transport momentum below and behind the animal in reaction to the lift force which both balances weight and provides thrust¹. Visualization of the vortex patterns enables the aerodynamic function of the flapping wings to be determined^2–6. Here we present the results of experiments with small vespertilionids which allow the first description of the vortex wake of bats. The aerodynamics of flapping flight is similar between birds and bats: thrust is always generated during the downstroke, but wingbeat gait (the cyclic pattern of wing movements) and the mechanical function of the upstroke are determined by wing morphology. We present the first evidence from aerodynamic experiments that in an individual bat or bat species, upstroke function and wingbeat gait also vary with flight speed, with aerodynamic lift being generated during the upstroke at high speeds but not during slow flight. This result confirms that flapping gaits are not species-specific, but are selected according to the mechanical conditions experienced by the animal.