A small-sized clausidiid copepod was found associated with the commercially important bivalve, Ruditapes philippinarum, from a Korean coastal lagoon. Since it resembles morphologically a European form known as Hersiliodes latericius, it is attributed to a new species, H. exiguus n. sp., of the same genus. The uneasy distinction between the genera Hersiliodes and the very similar Hemicyclops is discussed.