Larval shell morphology of four Cretaceous inoceramid species, Inoceramus tenuistriatus, Inoceramus hobetsensis, Inoceramus amakusensis and Sphenoceramus naumanni was examined on the basis of well-preserved specimens from the Yezo Group in the Tappu area, northwestern Hokkaido. SEM observations reveal that all of the four species possess large prodissoconchs with a similar shape. Relatively small prodissoconch I and large prodissoconch II indicate that they all underwent a long planktotrophic larval stage. This result as well as the available biogeographic data in the north Pacific Bioprovince suggest that these four species had a high dispersal capability in the larval stage.