The existence of waves in oceanic plankton populations has been conjectured on theoretical grounds. We report the first open ocean observations of a possible plankton wave in the Indian Ocean, seen in satellite ocean colour data. The chlorophyll feature travels from west to east, in the opposite direction to the mean flow and the Rossby wave and eddy propagation direction. For the strongest observed event, in February to April 1999, the wave propagates in the latitude band ~22--30¿S away from Madagascar and dies out at ~75¿E.