A underlying mechanism for two westward propagating tropical intraseasonal oscillations is discovered. Destabilization of equatorial normal modes by moist feedbacks, in particular, by evaporation-wind feedback, is responsible for both of them. The westward propagating 4-5 day oscillation and the quasi-biweekly oscillation result from destabilization of the mixed Rossby gravity (MRG) wave by the evaporation-wind feedback. The frequency and scale-selection of the unstable MRG wave depend critically on the background mean wind. In mean easterlies (as in the Pacific and the Atlantic), it results in the 4-5 day oscillation while in mean westerlies (as in the Indian Ocean during northern summer) it results in the quasi-biweekly mode. ¿ American Geophysical Union 1992