The horizontal and vertical distribution and diel migration of chaetognaths were investigated in the Kuroshio warm-core ring (KWCR) and adjacent waters during two cruises in August and September 1987. Four genera, with a total of 19 species, were identified from the KWCR and adjacent waters. The chaetognaths were most abundant in the marginal areas including fronts, and decreased rapidly from outside toward the center; four water types characterized by different species assemblages, were recognized. The horizontal distribution of epipelagic species was limited by fronts between the KWCR and the Oyashio water or a warm streamer. Sagitta scrippsae, neritic species and warm-water species are carried from the marginal areas to the central core water by the clockwise flow of the surface layer above the seasonal thermocline. Sagitta elegans and Eukrolmia hamata reach the epipelagic layer of the central water by vertical migration from the intermediate water existing under the core water. Warm-water species, S. elegans and E. hamata cannot survive in the central core water because they cannot adapt to the changes in temperature and salinity. S. scrippsae, S. minima and S. nagae are suited for living in the KWCR, but their normal breeding is controlled by food conditions.