The main objects in this survey were to find out whether the Japanese mice, Mus molossinus Temminck and Schlegel, live out in the several environments all the year, and whether they have a breeding season in such different habitats as house, straw stack, cultivated field and waste land. For this reseach the capture of the mice, confirmation of their nest holes, examination on the reproductive status and feeding habit were all carried out in Fukuoka District between June 1955 and April 1960 (Figs. 1-5). The trapping for capture of the mice was all done with break-back traps and cage traps baited with a dumpling made of wheat and buckwheat flours or a piece of cheese. The methods I have used for information on the reproductive status were the examination of implantation scars, corpora lutea, presence or absence of embryos and stage of oestrus cycle as determined by vaginal smears regarding the female, on the other hand, spermatozoa in smears of epididymides regarding the male. The results are shown in Tables 1-8 on the basis of the examination on 929 mice (females 402 ; males 403 ; young and subadults 124) and on nest holes from all kinds of environments : The mice living in the house take groceries as the food, and breed from spring to autumn. But it is clear that very little breeding occurs in winter. In the farm-house, compost and straw stacks adjoining farm-gardens or grass- and weed-grown areas mice were known to have moved from inside to field owing to the feeding. Such the first movement from inside to field occurred in spring, early summer and fall, and the reverse movement was in winter and rainy season. However the mice dwelling in the urban premises are confined to houses all the year. The food of the mice dwelling in the straw stacks is unhulled rice in winter. Straw stacks in this country are built in the end of November after harvest and these are removed by June of the following year. There is, therefore, the breeding season during winter and spring of the following year. At the beginning of winter the mice living in the fields migrate into straw stacks for the winter season. During autumn the mice are found both around straw stacks adjoining the farm-garden and at considerable distances away from farm-garden. The latter probably lives quite independently in the fields. Since the food has been eaten out at the beginning of spring, they moved from straw stacks to the cultivated field. At the end of spring (the rice planting season) mass of the mice in the straw stacks and the other places spread into the dry slopes of hillacks, high banks of paddy fields and farm-gardens. Most mice living in cultivated field live mainly in the farm-garden and marginal banks of paddy fields in summer. They are living on crops, weed seeds and blades. They can live without the shelters such as houses, compost and straw stacks in winter. They live in burrows in the ground at all seasons. Although they seem to have breeding season all the year round, the rates of fecundity in spring and autumn are higher than in the other seasons. The mice living in the waste land live in burrows in the ground under the weed-grown areas and banks, and they are living largely on weed seeds and blades all the year. They seem to have restricted breeding seasons in spring and autumn.
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