Observations of water vapor with high temporal and spatial resolution and good horizontal coverage just above the tropical tropopause have been scarce, but a preliminary version of such data has been developed using radiance measurements of the Microwave Limb Sounder. These data reveal distinct variations with periods in the ranges 10--25 days and 30--70 days, consistent with (respectively) slow Kelvin waves and the tropical intraseasonal oscillation. ¿ 1998 American Geophysical Union