The evolution of mixed layer temperature (taken as sea-surface temperature, SST) in the western Indian Ocean north of 20S and west of 80E during the First GARP Global Experiment (FGGE), 1979 is described and modelled. The FGGE-year development in time and space of SST is compared to the appropriate climatology. FGGE events occurred in phase with climatology, but some amplitude anomalies were observed. Heat budget computations for the surface mixed layer indicate that over 25% of the region studied energy fluxes through the sea surface can account for 80% of the observed SST variance. South of the equator, 80% of the variance is accounted for in 36% of the area and north, only 11%. Exceptions are noted along the western boundary, in the central and eastern Arabian Sea, and in a band south of the equator between 6S and 12S, east of 60E. The addition of entrainment through the base of the mixed layer improves the heat budget estimates over most of the region, in particular, along the Arabian coast. Near the northern part of the coast of east Africa, however, inclusion of the effect of horizontal advection gives more improvement. The breakdown of the heat budget computations in the central and eastern Arabian Sea and in the band south of the equator is attributed to a small signal in SST variance and few data in the regions.