Tropical cyclone rains have distinctly lower stable isotope ratios than rains in other tropical and summer precipitation systems, with a mean value slightly above that of water vapor near the sea surface. The isotope ratios also decrease radially inward to the eye wall, but appear to be anomalously low even near the periphery of the rain shield. These findings indicate that tropical cyclones are highly efficient precipitation systems and suggest the use of stable isotope ratios as dynamic tracers of a tropical cyclone's water and energy budgets. ¿ American Geophysical Union 1996