A 26-day oscillation in sea surface temperature (SST) data is observed in the western Indian Ocean, from 52¿ to 60¿E and in the vicinity of the equator. The SST data used in this study are obtained from the NOAA 9 satellite and are for the years 1987 and 1988. This fluctuation of SST at a period near 26 days is found to be antisymmetric about the equator and is trapped within the equatorial waveguide (equator ¿6¿). The variance associated with this oscillation has a maximum located at about 3¿ latitude; furthermore, the variance decreases at a faster rate toward the equator than poleward. These characteristics are consistent with the latitudinal structure for the mixed Rossby-gravity (or Yanai) waves as predicted from linear wave theory. The temporal variation of this 26-day oscillation is most energetic during the summer season (July to September), with maximum values of 0.4 ¿C and 0.8 ¿C found during August of 1987 and 1988 respectively. This observation agrees with the temporal variation of Yanai waves inferred from drifting buoy observations and numerical studies of the Indian Ocean. Thus we conclude that the Yanai wave is responsible for the 26-day fluctuation observed in the SST data in this region. ¿ American Geophysical Union 1992