The diurnal variation of lightning activity on and around Java Island, Indonesia, is examined by using the observation results from a magnetic direction finder network. The magnetic direction finder network has four direction finders on Java Island and has been operating since 1994. Lightning is active during the rainy season, usually from October to May. The highest ground flash density on Java Island was 16 flashes per km2 per year. The diurnal variation of the lightning activity over land shows its peak at around 1500 LT. There is a small peak in the early morning around 0300--0500 LT over the sea, several hours prior to the reported peak times of the cold cloudiness, an indicator of deep convection observed from meteorological satellites. Existence of close sea-land interaction related to the activity of deep convection in this area is suggested from the diurnal variations in the lightning activity. The diurnal variation is also found in the peak currents of negative first return strokes over land, medians from 25 kA around noon to 30 kA around midnight. The lightning activity over the ocean does not show such diurnal variation and delivers currents of higher amplitudes with a median of 36 kA. ¿ 1999 American Geophysical Union