This study examines the ecology and thermal regime in the Živa Voda anchialine cave on Hvar Island (Adriatic Sea, Croatia). The cave, which has no direct connections with the open sea, contains a dense population of the deep-sea hexactinellid sponge Oopsacas minuta Topsent, 1927. Several species of polychaetes, molluscs and crustaceans were also recorded. Two-year temperature data recorded simultaneously at two depths within the cave and at a nearby outside site provide information on thermal conditions in the system. The temperature oscillations within the cave are strongly dampened – varying from 14.6 °C in winter to 17.9 °C in summer – the annual range being four times smaller than in the adjoining sea. These data suggest that O. minuta can withstand higher temperatures than previously expected from deep-sea or cold-water cave occurrences, and that other environmental factors must be playing an important role.