Relative sea level change occurred along the western coast of the Istrian peninsula in Croatia. Sediments from two cores located in the bay of Santa Marina and Soline Bay have been analyzed for microfaunal and sedimentological investigations. Chronostratigraphical evidence was obtained from 14C dating of lagoonal shells (gastropods). From the sedimentological and paleo-environmental analysis, 4300 years of continuous deposition in a shallow marine environment was determined. At around 550 cal BP, marine sedimentation changed to continental. Consequently, the initial sedimentation rate of 0.7 mm/y changed to 2.36 mm/y after 550 cal BP. Those upper deposits of continental origin are today partly submerged. The data were further compared with a geophysical model which allows the prediction of the local sea level curve. As the results from the cores are largely below the curve, tectonic subsidence of the area is confirmed. Radiocarbon analysis allowed the age of the shells to be related to the age of the submerged tidal notch that occurs almost continuously some hundreds of meters south-eastwards, from the Gulf of Trieste to the Dalmatian coast.