The occurrence of a natural hybrid swarm between Hemerocallis citrina v. vespertina (Hara) M. Hotta and two varieties of H. fulva, i. e. , v. longituba (Miq.) Maxim. and v. disticha (Donn) M. Hotta, was first reported by Nakao and Yamashita (1956) from Matsunoki, located at the distance of 4 km east of Takayama City, Gifu Prefecture, Central Honshu.Based on the results of their investigation, it was concluded that this particular hybrid swarm found in a valley near Takayama represents an initial stage of introgression of genes of H. citrina v. vespertina into the population of H. fulva (Nakao and Yamashita, l. c. ) . Another example of introgressive hybridization in Hemerocallis, i. e., between H. middendorffii and H. yezoensis was reported by Kawano (1961) from Otanoshike near Kushiro City in Hokkaido. In both cases, the ecological disturbance of the habitats was deemed to be the maj or causes in breaking down the isolation barrier previously existing between these plants.In the present study, a natural hybrid population of Hemerocallis in the Matsunoki district was re-investigated from several new standpoints in order to know more precisely about the biological status of this particular population. In addition to the analyses of gross morphology and ecology, the karyotypes and meiotic behaviours of the chromosomes were studied. The pollen fertility, the setting rate and germination rate of the seeds were also examined. Furthermore, the electrophoretic banding patterns of acid-soluble protein extracted from the rhizomes of both parental and hybrid plants were likewise analyzed and compared with one another
