Observations of a fifty-day oscillation in the meridional component of the South Equatorial Current in the Western Indian Ocean are presented. Currents measured in 4000 m of water close to the atoll of Aldabra (46-degrees-E, 9-degrees-S) reveal the oscillation throughout the water column to a depth of 3 000 m. Further evidence of the oscillation is presented in shipboard Acoustic Doppler Current Profiler (ADCP) sections and in Coastal Zone Color Scanner (CZCS) imagery. The oscillations are consistent with a zonally propagating Rossby wave caused by shear instability in the zonal flow of the South Equatorial Current.