Meridional transport and origin of North Pacific Intermediate Water (NPIW) in the Kuroshio-Oyashio interfrontal zone (K/O zone) are examined using hydrographic and lowered acoustic Doppler current profiler (LADCP) velocity data along 37¿N obtained in July 1998. An optimal interpolation method was applied to objectively derive transports and their spatial-interpolation errors. A statistically significant northward transport of 12.0 ¿ 4.6 Sv was observed in the density interval 26.6--27.5σθ between the coast (141¿E) and the Shatsky Rise (157¿E). In the upper part of NPIW (26.6--27.0σθ), the total transport was 4.1 ¿ 1.2 Sv, of which the Kuroshio component was 3.0 ¿ 0.7 Sv and Oyashio was 1.1 ¿ 0.5 Sv. In the lower NPIW (27.0--27.5σθ), the total transport was 7.9 ¿ 3.4 Sv, comprising 4.7 ¿ 1.9 Sv of Kuroshio water and 3.2 ¿ 1.5 Sv of Oyashio water. A southward transport of 4.4 ¿ 3.6 Sv was observed between the Shatsky Rise and 179.5¿E in the density interval 26.6--27.5σθ. In the region west of the Shatsky Rise, the Oyashio mixing ratio derived from temperature and salinity in the K/O zone is larger than the ratio of Oyashio to Kuroshio transport, implying southward transport of 1 Sv of Oyashio water across the Oyashio Front in the upper NPIW. Mixing and transport ratios are almost the same in the lower NPIW, suggesting that this region is maintained only by the northward transport of NPIW formed near the Kuroshio Extension.