Regional and interannual variations of the rainy season over Indonesia are investigated using daily rainfall data during 1961-90. Pentad-mean rainfall data, with a relatively better continuity have been obtained for 46 stations, and the annual and semi-annual cycles of rainfall at these stations have been objectively analyzed by harmonic analysis. The onset of southern-hemispheric spring/summer (SON/ DJF) rainy season starts from the Indian Ocean side of Jawa in the middle September, and propagates northward (in Jawa) and eastward (to Nusa Tenggara in middle December). Another route of rainy sea-son propagation from Irian Jaya to Nusa Tenggara is observed. The withdrawal of the rainy season starts from western Nusa Tenggara in March, and goes eastward (to eastern Nusa Tenggara), and westward (to Jawa), until late May.
Rainy season onset comes later (earlier) in El Niño (La Niña) years than the average at most stations (particularly in the south-eastern part of Jawa). Correlations between rainfall amounts at those stations, and the southern oscillation index in SON, are significantly high. However, the rainfall amount throughout a rainy season is not dependent upon the length of the rainy season (between onset and withdrawal) in many areas (except southern Sulawesi). The dominant time scale of interannual variations of rainy season onset during 1961-90 is 2-3 years, which looks somewhat shorter than that during 1910-41.