We have developed a long-term observation system for the study of halocline behavior in brackish lakes and estuaries. The system consists of an acoustic profiling unit, a digital recorder, and a digital Play back system. By using this system, we have observed halocline behavior in the center and at the margin of Nakaumi lagoon, southwest Japan. Salinity, pH, dissolved oxygen, water temperature, and water flow rate and direction were checked by the other equipment. The results suggest that the halocline is easily moved. and its behavior is influenced by wind.