We have investigated the seasonal variation of the salinity in the mixed layer of the Indonesian Seas from a Lagrangian viewpoint using the ''particle-tracking method.'' Numerous labeled particles are tracked in a velocity field obtained using the robust diagnostic general circulation model with seasonal forcing. Salinities tagged to these particles are subsequently changed by freshwater flux through sea surface during their transit. Finally, the salinity in the ''mixed layer'' (upper 50 m) of each basin comprising the Indonesian Seas is calculated by monthly averaging the salinities of all particles located in the mixed layer of each basin. The seasonal salinity variation in the mixed layer obtained with this method shows similar features to that observed. Since this method has the advantage of clarifying the origins and histories of waters affecting the salinity variation in the mixed layer, a close examination of the result by the particle-tracking method revealed the major factors causing seasonal salinity variation in each basin of the Indonesian Seas. Seasonal variation of the salinity in the mixed layer of the Indonesian Seas is determined by the seasonal variations of net air-sea freshwater flux, three-dimensional velocity field, and their combined effect. ¿ American Geophysical Union 1996