The effective environmental half-lives of Cs137 and Sr90 together with their variations in concentrations in the coastal seawater of Japan were examined based on results of long-term and systematic measurements during 1984--1994. Concentrations of these radionuclides in surface water from 12 areas in the region of the Kuroshio and the Tsushima Currents along Japan were similar and higher than in surface waters of the cold current of Oyashio. Relatively constant concentrations were observed in seawater above 200 m depth that appears to be well mixed, as generally assumed, with sharp gradient below 200 m depth. The observed mean concentrations of Cs137 in coastal waters of Japan decreased from 3.1¿0.2 mBq/L in the surface mixed layer (0--200 m depth) to 1.8¿0.2 mBq/L at 500 m depth (a decrease of 58%). The observed mean concentration of Sr90 in coastal waters of Japan decreased from 2.2¿0.2 mBq/L at the surface mixed layer to 1.4¿0.2 mBq/L at 500 m depth (a decrease of 64%). The vertical distribution of Cs137/90Sr activity ratio ranged from 1.45 at surface to 1.31 at a depth of 450--500 m, suggesting a possible difference in the behavior between Sr90 and Cs137 in coastal seawater. Concentrations of Sr90 and Cs137 in the surface mixed layer were 3.3¿0.4 mBq/L (n=40) and 4.3¿0.5 mBq/L (n=40), respectively, in 1984. The concentrations dropped fairly steadily from 1984 to 1994, except for a clear increase of Cs137 in the spring of 1986 (5.0¿1.1 mBq/L, n=34) due to Chernobyl deposit. The concentrations of Sr90 and Cs137 in 1994 were 2.2¿0.2 mBq/L (n=51) and 3.1¿0.2 mBq/L (n=51), respectively. Effective environmental half-lives of Sr90 and Cs137 of the surface mixed layer in the Tsushima Current system are calculated to be 17.3 years (95% confidence interval 16.1--18.7 years) during 1984--1994 and 18.7 years (13.4--31.2 years) during 1987--1994, respectively. ¿ 1998 American Geophysical Union