Six taxa of cassidulinid foraminifera were obtained from the Seoguipo Formation, Jeju Island, Korea. The cassidulinid assemblage is characterized by the abundant occurrence of Cassidulina norvartgi Thalmann, and in spite of small numbers of individuals it contains a few Pacific forms such as Paracassidulina minuta (Cushman) and Globocassidulina venustas Nomura. Such an assemblage is considered to be transitional between the Japan Sea and the Pacific Provinces.