[en] The stable isotope content of waters collected from the investigated area provided important information about the unusual underground ecosystem of Movile Cave that is based on chemoautotrophic conditions. As a conservative tracer in carbonate rocks deuterium was used to determine the un-elucidated problems of the water origin in cave. The cave developed into hydrogeologic complex of South-Dobrogea and is located near the Mangalia City. Rocks classified according to their hydrological function include the unconfined aquifer in the Sarmatian karstic limestones and the confined aquifer in the Mesozoic limestones which are fractured and karstic and represent a unitary complex interconnected with upper lying aquifer. The tectonic structure indicates three faults: a fault orientated by NS lineament intersects the Kara Oban Fault orientated on NW-SE direction and the Movile-Aurora Fault in the Mangalia Swamp. The values of δ²H and δ¹⁸O plotted on GMWL (Craig line) indicate the meteoric origin of groundwater in Movile Cave area. The average value of δ²H for all samples is -85 ⁰/₀₀, for the most samples (76%) the δ²H values are lower than -80 ⁰/₀₀ and the water is isotopically more light than the waters from the semiarid lowland of the Dobrogea district (over -76 ⁰/₀₀). The δ²H-Δdens correlation of samples delineated three isotopically water types: groundwater with low deuterium content (< -88 ⁰/₀₀), water with deuterium content of about mean value and waters with higher δ²H (> -78 ⁰/₀₀). Time series of the deuterium concentration from groundwater samples both from the aquifer in Mesozoic limestone and from the unconfined aquifer in Sarmatian limestone exhibits a distinct seasonal effect, indicating a strong link with isotopic composition of precipitation, but a lower deuterium concentration in summer than in winter. This six-month phase shift of the isotopic composition variations with respect to normal succession of seasonal maximum and minimum values of precipitation and lower δ²H average values are due to the contribution of the watershed of the more elevated inland (from the Balkan Plateau) during summer. The study indicates that the intrusion of water in the karst occurs by conduit flow controlled by the hydrostatic pressure and the geometry of conduit. The hydrogeology of region favours these findings. The interconnection between aquifers is pointed out by the correlation between the δ²H values and total salinity content. The water of Movile Cave is the result of the mixing of two end members that have different hydrogen isotope ratios. The spatial variation of the deuterium content delineated the areas with vertical inflow and the flow path of groundwater from deep aquifer. The discharge of the deep aquifers occurs through faults and karstic channels into overlying aquifer. The Movile Cave and Karaoban Lake are connected and the cave water discharges by an overflow mechanism that offers to cave the isolation from the atmosphere. (author)