Prunus yedoensis Matsumura (Somei-yoshino) is the most famous flowering cherry tree grown in Japan. However, its origin has been unknown. This species has abundant beautiful flowers, but sets only a few seeds. Accordingly, it can be propagated only by grafting. However, it grows more rapidly than any other cherry tree. From these facts, I have assumed that it might be a hybrid. I gathered seeds from trees of this species in 1952. The seeds were sown in 1953, and the seedlings were observed from 1954 to 1962. From these observations P. yedoensis was considered to be a hybrid between P. lannesiana var. speciosa (Oshima-zakura) and P. subhirtella var. pendula form. ascendens (Edo-higan), whose characteristics differ as follows: speciosa is distinguished from ascendens by underneath glabrous and larger leaves, and vigorously growing stems. The seedlings of yedoensis showed in this respect a series of intergrades ranging from speciosa to ascendens type. Some of the seedlings bloomed in the spring of 1958, and since then, 25 trees had flowers. In many flower characters, viz., size, color, and hairiness at peduncle, receptacle, calyx, style and ovary, these trees showed a wide range of variation from speciosa to ascendens. In further experiments carried out since 1957, I made reciprocal crosses between speciosa and ascendens. In 1961, 8 hybrid plants bloomed. They were intermediate between the parents in the characters of stem, leaf and flower, and appeared as a whole to be similar to one another, though they showed some minor differences. Two of them had hairs on the style and ovary like P. yedoensis, and the other two had a few hairs, while the rest were hairless.