Cave diving expeditions were conducted in the Golfo di Orosei in June/July 2003 and July 2004. The main site of the exploration was located 500 m north of Cala Sisine (Fig. 1). The cave system of Grotta del Bel Torrente is one of the biggest in Sardegna. To date, a total of more than 2.5 km dry and water-filled passages have been surveyed. A map of the cave was generated as the basis for further investigations. The focus of the biological work is on the diversity of the species and their distribution throughout the cave until 800 m from the entrance. Until 350 m from the entrance a clearly visible halocline is developed between the upper layer (20 °C, salinity about 22) and the underlying water column (22 °C, salinity about 33). A zone of intermediate water salinity and temperature, without halocline, follows over a distance of 100 m. In more than 450 m from the entrance the salinity drops to 1 and temperature is 15 °C. Up to 350 m from the entrance, marine sponges, molluscs, crustaceans, polychaetes, ascidians and fishes were observed. The occurrence of empty tubes of Serpulidae and shells of oysters in this zone may reflect oxygen deficiency. Some fish such as mullet (Liza sp.) migrate between the fresh and salt water columns.