Reproduction in Apodemus semotus was investigated under laboratory and field conditions. Fifty-six litters consisting of 240 offspring were born from 13 paired females in the laboraroty. Litter size was 4.29±0.50 (range: 2-7). The number of the first litter (3.78±1.20) tended to be smaller than the second (4.00t1.16). The gestation period was estimated at 20 days on the basis of the shortest interbirth interval between successive litters. Data on reproduction in the field population were based on 359 specimens. The embryo count ranged from 1 to 5, being 3.49k0.94 on average. In general, the litter size in A. semotus was influenced by parity and was significantly larger in the laboratory colony than in the field population. Males with body weight less than 22 g had no sperm in the cauda epididymis, whereas males weighing 22 g or more were in the most active stage of spermatogenesis in both spring and autumn as well as in the time of female breeding activity, either sexually active or potentially so in summer, and regressive in spermatogenesis in winter. Thus, the seasonal breeding pattern of A. semotus was bimodal with the two peaks of reproductive activity in both spring and autumn, the depression caused by the yearling adults entering into the population in summer and the cessation due to xeric conditions together with low temperatures.