Results of systematic long-term measurements of 90Sr and 137Cs in surface seawater, performed at four locations along the Croatian coast of the Adriatic Sea, are summarized. Observed trends in annual mean activity concentrations are discussed. 90Sr and 137Cs fallout activities affect seawater activity, the coefficient of correlation between 90Sr fallout activity and 90Sr seawater activity being 0.72. No significant variations of 90Sr mean residence time in the sea-mixed layer on different locations were found. From 1978-1985, the 137Cs:90Sr activity ratio in seawater was 1.52 +/- 0.40. In May 1986, as the consequence of the Chernobyl nuclear accident, this radio increased to 92.4 +/- 58.2.