Oxygen and deuterium isotopes and major-ion chemistry of water from a large karst spring were used in an attempt to decipher water recharge, transmission, and storage characteristics of a karst aquifer system. Ionic concentrations and isotopic data indicated that the bulk of discharge during peak flow was derived from groundwater storage. Isotopic hydrograph separation of storm flow revealed that maximum rainwater contribution to discharge was 18 to 24 hours after peak flow and rainwater contributed 20 to 25% of spring discharge over the monitoring periods. Water released from phreatic and vadose conduit storage may have contributed to discharge with the onset of storm flow, while water from soil moisture and epikarst storage may have arrived during initial discharge recession. ¿ American Geophysical Union 1996