A word “possible” is used in the title, as QUAYLE (1931) did in the relation of Mediterranean fruit fly to the United States, because the distribution of both flies in Japan, etc. will be possible only in so far as the temperature and host conditions are favorable. Even if there were chances of being introduced from Formosa, Okinawa and other tropical countries, the melon fly (Dacus cucurbitae COQUILLETT) and the Oriental fruit fly (Dacus dorsalis HENDEL) have never been found in Japan, Korea and other adjacent northeastern countries. In order not only to search for the reason why they have not survived in the temperate countries but also to find out whether they can establish there in future, experimental researches were undertaken on the followingitems:<br>a. Ranges of effective and favorable temperatures for the development and the reproduction, <br>b. Developmental and reproductive zeros, <br>c. Total sums of effective temperatures for the development and the reproduction, <br>d. Least exposures to cold lethal for 100 per cent of the insects in all stages, <br>e. Longevities of larvae and adults deprived of food in low temperatures, <br>f. Discussions of the obtained data in comparison with the actual distribution and abundance of the flies in the world.<br>All the temperatures treated were constant, varying and gradually falling ones actually found in the cold seasons of the temperate climates.<br>Abbreviations: A: adult flies, E: eggs, G: gradually falling temperatures, K: constant temperatures, M: maggots, P: pupae, V: varying temperatures.<br>The present report includes the results of items a, b and a part of f.<br>1. Temperatures concerning development and reproduction determined experimentally are shown in the accompanying table.<br>2. The favorable temperatures for the development and the reproduction shown above tell very well the actual status of the distribution and abundance of both flies as well as the natural infestations of fruits and vegetables in the world.<br>3. It was suggested from the experiments of favorable temperatures that, because the mean temperatures of the coldest month are lower than 8°C and those of the warmest one are higher than 16° in Japan and in the neighbouring countries, the insects in all the stages will either become inactive or fall into cold death in winter of these countries, though in warm seasons the development and the reproduction may take a place.<br>4. Agreement was found in regard to the relation between the developmantal and the reproductive zeros and the actual distribution of both species in the globe.<br>5. The development and the reproduction of both flies will cease in winter in the areas north of the line connecting Hachijo Is. to Tokara Is., though in the southern islands such as Hachijo, Ogasawara, Okinawa and Formosa they can no doubt live throughout a year, for the isothermal line of January temperatures of 10°C runs completley coinciding with the line above mentioned.