New species after new species has been discovered of black flies in Japan, adding to the knowledge in this insect. On Mt. Daisen of Tottori Prefecture, we obtained specimen believed to be a new species belonging to the subgenus Eusimulium, and its morphology has been studied. The pupa of this species was discovered in 1955 by Yoshida, Sato and Shogaki. It occurs in very small numbers at limited areas of high mountains. Only pupae having been collected, the characteristics of the adult was unknown until Okamoto in 1957 succeeded in raising adults from pupae he had collected. The morphology of this species has thus been clarified. This new species is now named "Simulium (Eusimulium) kobayashii" after Dr. Harujiro Kobayashi. The writers express their gratitudes to Dr. H. Kobayashi and Prof. Dr. M. Nagahana for their directions on the present studies. Acknowledgement with thanks is also expressed for the assistances given by Mr. K. Kamei, Mr. N. Matsubara, Miss K. Nishio, Mr. Z. Sugihara, and Mr. S. Oda.