The gestation period was approximately 21 days, the same as reported for _M. pennsylvanicus_ (Bailey, _loc. cit._; Hamilton, 1941:13) and _M. californicus_ (Hatfield, 1935:264). A more precise study of the breeding habits of _M. ochrogaster_ failed to materialize when the voles refused to breed in captivity. Fisher (1945:437) also reported that _M. ochrogaster_ failed to breed in captivity although _M. pennsylvanicus_ (Bailey, 1924) and _M. californicus_ (Hatfield, 1935) reproduced readily in the laboratory. Litter Size and Weight In the course of this study 65 litters were observed. The mean number of young per litter was 3.18 +- 0.24 and the median was three (Fig. 10). Three litters contained but one individual and the largest litter contained six individuals. Other investigators have reported the number of young per litter in _M. ochrogaster_ as three or four (Lantz, 1907:18) and 3.4 (1-7) (Jameson, 1947:146). _M. pennsylvanicus_ seems to have larger litters. Although Poiley (1949:317) found the mean size of 416 litters to be only 3.72 +- 0.18, both Bailey (1924:528) and Hamilton (1941:15) found five to be the commonest number of young per litter in that species. Leslie and Ransom (1940:29) reported the average number of live births per litter to be 3.61 in the British vole, _M. agrestis_. Selle (1928:96) reported the average size of five litters of _M. californicus_ to be 4.8. Hatfield (1935:265), working with the same species, found that litter size varied directly with the age of the female producing the litter. He reported litters of young females as two to four young per litter and of older females as five to seven young per litter. In the litters of _M. ochrogaster_ that I examined, young females did not have more than three young and usually had but two. However, older females had litters of one, two and three often enough so that no relationship, as described above, was indicated clearly. [Illustration: FIG. 10. Distribution of litter size among 65 litters of voles.] No seasonal variation in litter size was noted. The mean size of the litters in 1950, 2.68 +- 0.30, was significantly lower than that found in 1951 (3.76 +- 0.20) but neither differed significantly from the mean size of litters in 1952 (3.35 +- 0.66). The lower mean size of litters was in part coincidental with a high population level and the higher mean of the two later years was in part coincidental with a low population level. Since a sharp break in the curve for population density occurred after the flood in July, 1951, the litters were arranged in pre-flood and post-flood categories for study. Pre-flood litters averaged 3.07 +- 0.28 young per litter whereas post-flood litters averaged 3.34 +- 0.48. This difference was not significant. Increase in litter size, if it had actually occurred, might have been a response to the increasing food supply and lower population density after the flood.