Temperature variations in the tropical lower and middle stratosphere are influenced by at least five distinct driving forces. These are (1) the mechanism of the regular seasonal cycle, (2) the quasi-biennial oscillation (QBO) in zonal winds, (3) the semiannual zonal wind oscillation (SAO) at higher leveals, (4) El Ni¿o-Southern Oscillation (ENSO) effects driven by the underlying troposphere, and (5) radiative effects, including volcanic aerosol heating. Radiosonde measurements of temperatures from a number of tropical stations, mostly in the western Pacific region, are used in this paper to examine the characteristic annual and interannual temperature variability in the stratosphere below the 10-hPa pressure (~31 km) over a time period of 17 years, chosen to eliminate or at least minimize the effect of volcanic eruptions. Both annual and interannual variations are found to show a fairly distinct transition between the lower and the middle stratosphere at about the 35-hPa level (~23 km). The lower stratosphere, below this transition level, is strongly influenced by the ENSO cycle as well as by the QBO. The overall result of the interaction is to modulate the amplitude of the normal stratospheric seasonal cycle and to impose a biennial component on it, so that alternate seasonal cycle are stronger or weaker than normal. Additional modulation by the ENSO cycle occurs at its quasi-period of 3--5 years, giving rise to a complex net behavior. In the middle stratosphere above the transition level, there is no discernible ENSO incluence, and departures from the regular semiannual seasonal cycle are dominated by the QBO. Recent ideas on the underlyin physical mechanisms governing these variations are discussed, as is the relationship of the radiosonde measurements to recent satellite remote-sensing observations. ¿ American Geophysical Union 1994