Numerical analyses of seiche and continuous measurements of water level and groundwater seepage rates into Lake Biwa, Japan, were made to evaluate interactions between surface and subsurface waters. An automatic seepage meter was used to obtain continuous groundwater seepage rates. Analyses of the fluctuations of seepage rate and water level in the lake were done using the fast Fourier transform method. The finite element method was used for analysis of the seiche in the lake. Dominant periods of fluctuations of the groundwater seepage rate into Lake Biwa agree well with those of fluctuations of water levels in the lake. The dominant fluctuations of the groundwater seepage rate are caused by the seiche in Lake Biwa. ¿ American Geophysical Union 1996