We have used a simple, one-dimensional model to simulate the diurnal cycle of the equatorial upper ocean. The model is initialized with the stratification and shear of the Equatorial Undercurrent (EUC) and is driven with heating and wind stress. A surface mixed layer is determined by bulk stability requirements, and a transition layer below the mixed layer is simulated by requiring that the gradient Richardson number be no less than 1/4. A principal result is that the nighttime phase of the diurnal cycle is strongly affected by the EUC, resulting in deep mixing and large dissipation at night consistent with observations of the equatorial upper ocean during the 1984 Tropic Heat experiment. The daytime (heating) phase of the simulated diurnal cycle is very similar to that seen at mid-latitudes. Solar heating produces a stably stratified surface layer roughly 10 m thick within which there is little, O(3¿10-8 W kg-1), turbulent dissipation. For the typical range of conditions at the equator, diurnal warming of the sea surface is 0.2¿--0.5¿C, and the diurnal variation of surface current (diurnal jet) is 0.1--0.2 m s-1, consistent with observations. The nighttime (cooling) phase of the simulated diurnal cycle is quite different from that seen at mid-latitudes. As cooling removes the warm, stable surface layer, the wind stress can work directly against the shear of the EUC. This produces a transition layer that can reach to 80 m depth, or nearly to the core of the EUC. Within this layer the turbulent dissipation is quite large, O(2¿10-7 W kg-1). Thus the simulated dissipation has a diurnal range of more than a factor of 5, as observed in Tropic Heat, though the diurnal cycle of stratification and current are fairly modest. ¿ American Geophysical Union 1992