Undaria pinnatifida (HARV.) SUR., U. undarioides (YENDO) OKAM. and U. peterseniana (KJELLM.) OKAM. are members of the genus of Undaria, Laminariales. They are confined to Japan and are useful algi as human food. In recent years, SAITO3) expereimented on possibility of artifical crossing among three species of Undaria, and succeeded to grow the hybrids in the sea. The present paper deals with the artifical crossing between U. peterseniana and U. pinnatifida. The results of studies are summarized as follows: Artifical crossing between U. peterseniana ♀ and U. pinnatifida ♂ were successful and many hybrids were obtained at the cultivation in the sea. The hybrids are intermediate between their parental species not ohly morphologically but also ecologically. All grown-up hybrids produced sporangial sori mainly on the sporophyll at the lowest margin of blade. Morphological and ecological aspects of the gametophytes from the hybrids are almost similar to those of the parental species. At the first nuclear division in the zoosporangia of U. pinnatifida, U. peterseniana and the hybrids, thirty chromosomes each were counted.
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