In 1955, with the assistance of 16 health centers in Niigata Prefecture, flies of medical importance were collected by means of a cage or ribbon trap throughout the year. They totalled 30, 751 specimens consisting of 18, 181 outdoor flies (15, 308, collected in an open field, plus 2, 873, trapped in such mountain areas as Kushigatayama, Ginzan-daira and Oze) and 12, 570 indoor ones. As shown in the classification table, they were classified into 10 families, 32 genera and 61 species. To describe the fly fauna, in family Muscidae, Muscina angustifrons held a superiority in every region. In family Anthomyiidae, both Fannia and Qphyra species were omnipresent, while Ophyra nigra was distributed more densely in a piedmont rather than in a plain. In Greenbottle flies of family Calliphoridae, Phaenica sericata ranked first in inhabitation number in most of the regions. A distribution of P. sericata >Lucilia illustris>L. caesar was observed in most of the region and that of P. sericata>L. caesar>L. illustris in some districts. An uncommon distribution of L. caesar>P. sericata was seen in Sanjo and Koide districts, that of P. sericata>L. caesar>P. cuprina, L. illustris in Niitsu district, and that of P. sericata>L. caesar>Hemipyrelia ligurriens>L. illustris in Sado Island. And L. papuensis, which is usually seen in a mountain area only, inhabited as a common species in Sado Island. Phormia regina and Protophormia terae-novae were found to inhibit in Naoetsu district. This may be of some help in the investigations of such an epidemic as polio. Of Sarcophagid flies, 14 species were discovered in Niigata Prefecture and showed a distribution of Sarcophaga peregrina>S. melanura>S. similis common to every district. It was interesting that S. tsushimae was discovered in Sado Island as well as in Tsushima, and S. polystylata originally discovered in Peiping was found out in a coastal plain along the Japan Sea. Concerning the fauna of indoor flies trapped by a ribbon, a distribution of Musca domestica>Fannia canicularis type was seen in some districts and that of the reverse type in others. Other species of flies were also found in small numbers. It, therefore, followed that many species of flies would invade indoors. Generally most of the flies of medical importance made three appearance in large numbers in spring, summer and autumn. Some species of family Calliphoridae, for instance, Calliphorinii, made their appearance in spring and autumn, and hid themselves in summer while Lucilia species are very common chiefly in summer, as flies of Sarcophagid flies did. Scopeuma stercorarium and Stenodryomyza formosa appears in early spring and late autumn, while Helomyza modesta in late autumn and early winter. Any summer mountains abunded with flies, but a fly fauna in them was fairly different from that in a plain. To enter into details, in both Kushigata-yama and Ginzan-daira, Chrysomya pinguis predominated in numbers; P. sericata hardly made its appearance; L. caesar was relatively numerous; L. papuensis was active as a common species in a mountain as its name suggests, and Calliphora lata and Aldrichina grahami, which, in a plain, do not usually make their appearance during summer, led an active life. Of family Muscidae, Muscina angustifrons was seen most frequently as a common species in a mountain region, and also M. assimilis and M. pabulorum were found easily. Of family Anthomyiidae, Ophyra nigra was distributed comparatively densely, and Hylemyia platura, which is distributed all over the country, was also discovered in the mountain areas. Scopeuma stercorarium and Stenodryomyza formosa were not trapped in those places. As for Sarcophagid flies, a distribution of S. peregrina>S. melanura>S. similis commonly observable in a plain, did not stand for the mountain areas, that is, a distribution of S. peregrina>S. albiceps>S. similis>S. me

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