The main objects in this study were to find out what kind of a breeding habit has the Japanese mouse, Mus molossinus Temminck and Schlegel. For this reseach I have tried to capture mice throughout the year. The trapping was carried out at farming houses, urban houses and cultivated fields in Fukuoka district from June, 1955 to May, 1957. The investigation has thrown some light upon this problem. The results are as follows : 1. The mouse breeds throughout the year under the natural environments. But the rate of pregnancy in spring and autumn is higher than in the other seasons. However, the rate is at a maximum in autumn. 2. The majority of female mice caught in spring (Mar.-May) and autumn (Sept.-Nov.) are fecund, but in summer (June-Aug.) and winter (Dec.-Feb.) is fecundity at a minimum. 3. Average number of normal foetus per litter in autumn is more than that in spring. 4. Percentage of atrophic foetus in spring is higher than that in autumn. 5. In female mice is found the seasonal variation in average weight of uterus, and it is at a maximum in autumn and at a minimum in winter. 6. Male mice have spermatozoa in the epididymis throughout y ear. 7. The average weight of testes, vesicular glands and coagulating glands is at a maximum in autumn and at a minimum in winter. Judging from the inspections on the reproductive organs, the sexual activity in male mice seems to be varied seasonally. From the data mentioned above I conclude that the Japanese mouse is able to breed all over the year, namely polyestrus, but in autumn the breeding activity is the highest, secondly in spring, thirdly in summer and the lowest in winter.
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