Source water used by plants of several species in a semi-evergreen lowland tropical forest on Barro Colorado Island, Panama, was assessed by comparing the relative abundance of deuterium, D, versus hydrogen, H (stable hydrogen isotope composition, Ξ΄D) in xylem sap and in soil water at different depths, during the dry season of 1992. Ecological correlates of source water were examined by comparing xylem water Ξ΄D values with leaf phenology, leaf water status determined with a pressure chamber, and rates of water use determined as mass flow of sap using the stem heat balance method. Soil water Ξ΄D values decreased sharply to 30 cm, then remained relatively constant with increasing depth. Average Ξ΄D values were-13β°, for 0-30 cm depth and-36.7β° for 30-100 cm depth. Soil water Ξ΄D values were negatively associated with soil water content and soil water potential. Concurrent analyses of xylem water revealed a high degree of partitioning of water resources among species of this tropical forest. Xylem water Ξ΄D of deciduous trees (average=-25.3Β±1.4β°) was higher than that of evergreen trees (average=-36.3Β±3.5β°), indicating that evergreen species had access to the more abundant soil water at greater depth than deciduous species. In evergreen shade-tolerant and high-light requiring shrubs and small trees, Ξ΄D of xylem water was negatively correlated with transpiration rate and leaf water potential indicating that species using deeper, more abundant water resources had both higher rates of water use and more favorable leaf water status.