We measured the in situ primary production at four stations from the surface to 80 m off Sanriku in late May 1997. The depth-integrated daily primary production in the upper 80 m was estimated to be 391, 468, and 855 mgC m-2d-1 in water from the Oyashio, and 336 mgC m-2d-1 in the warm-core ring. The variation in the primary production was primarily due to the variation in phytoplankton activity (chlorophyll a-specific primary production). A combination of previous and present studies in water from the Oyashio and the warm-core ring suggested that phytoplankton activity is proportional to light intensity between 12 and 50 Ein m-2d-1 which is close to the usual light condition (61-75 Ein m-2d-1) off Sanriku in May and June. Light may be a limiting factor for phytoplankton off Sanriku in late spring and early summer.