The isotopic composition of lead in the Japanese primary basalts gradually decreases in radiogenic character in a transverse from the Pacific Ocean side to the Japan Sea side, whereas the observed 238U 204Pb and 232Th 204Pb ratios in the basalts increase in the same direction. The isotopic composition of lead suggests that tholeiite (Pacific side) is generated at a shallower depth than alkali basalt (Japan Sea side) as postulated by Kuno. The inverse correlation of the 238U 204Pb with the 206Pb 204Pb ratios could be due to preferential Pb extraction with respect to uranium at shallow depths. Alternatively, these geochemical results can be interpreted in terms of the Pacific oceanic rigid plate being underthurst beneath the island arc. The isotopic variation in basalts across the Japanese island arc would result from different proportions of the plate material and the upper mantle of continental side in the partial melt. ?? 1969.