Peak rates of leaf fall almost always occur during dry seasons in low-latitude, low-elevation tropical forests. The hypothesis that plant water stress is the proximal cue for leaf fall was tested by augmenting water supplies during the 4-mo dry season in plots of tropical moist forest on Barro Colorado Island, Panama. The manipulation maintained soil water potentials at or above field capacity throughout the dry season but did not affect atmospheric conditions in the canopy. For most species, dry-season leaf water potentials in the manipulated plots were similar to wet-season values and both were consistently greater than dry-season values in control plots. The manipulation delayed leaf fall for 2 of 9 species of trees for which qualitative data area available and possibly for 2 of 20 species of trees and lianas for which quantitative data are available. Timing of leaf fall was indistinguishable in manipulated and control plots for the remaining 25 species. Plant water status was thus rarely the proximal cue for leaf fall. -from Authors