Based on the plankton samples collected in the Korea Strait in 1972, a study was conducted on the vertical distribution of chaetognaths in relation to water masses in th Strait. The settling volume of total plankton collected in the Strait ranged from 0.3 to 5 cc/10㎥ and showed a distinctive variation in the vertical distribution between day and night. The large amount of volume was found in the upper layer at night and deeper layer during the day time collections. A total of 19 species and one forma of chaetognaths were identified from the present samples. In general, the number of species and individuals of chaetognaths were abundant in the upper layer. But in August they were distributed almost evenly from the surface to the bottom layer. Particularly several species of warm water chaetognaths, i. e., Sagitta enflata and S. regularis appeared abundantly in the deeper layer in summer. This indicates a sinking phenomenon of warm water from the surface to the bottom layer. As for the vertical distribution of S.elegans, a cold water species, in the Korean Strait, it is restricted only to the bottom layer except in the region of upwelling where they appear in the middle layer. This species is usually distributed in the depth of below 150m in the southern part of Japan Sea(Park, 1970), and it is usually distributed as far south as the Strait between Busan and Tsushima. In addtion, cold water species of copepods such as Pseudocalanus minutus and Metridialucens appear in the western side of Thushima. As indicated by the vertical and horizontal distribution of S. elegans in the Strait, the cold water flows as an undercurrent along the bottom from the southern part of the Japan Sea to the Korea Strait between Busan and Thushima in summer and fall, with a trend of uprising along the coast of Korea. S. decipiens has been found only in the depth of below 50m except in the coastal area where they appear in the upper layer. Therefore the vertical distribution of this species can be used for tracing the occurrence of upwelling and the movement of water from the middle layer