The wake of a flexible flapping membrane has been investigated using flow visualization and particle image velocimetry. The aspect ratio of the flexible membrane was 0.27. The Strouhal number (St) was varied from 0.44 to 0.16, corresponding to Reynolds numbers between 2.2 × 10 5 and 4.1×10 5. Under these conditions, a wake structure was observed that resembles the 2P structure characteristic of flapping foils but containing at least four vortex pairs per cycle. For St ≥ 0.35, vorticity contour plots suggest that each multiple vortex pair structure has approximately zero net circulation; whereas for lower St, vorticity of one sign is diminished such that the wake begins to resemble the classical von Kármán vortex street. A mean wake velocity profile measured at the mid-span of the membrane suggests a net drag for all flow conditions considered, with the minimum drag occurring for St = 0.35. However, preliminary flow visualization experiments with an increased aspect ratio of 0.45 show a simpler 2S-type structure resembling a reverse von Kármán vortex street for which greater efficiency in thrust production is expected.