In other species of _Microtus_, so far as known, a 21-day gestation period seems to be the rule (Bailey, 1924:528; Hamilton, 1941:13; Hatfield, 1935:264). _M. ochrogaster_ seems to conform to this pattern, but the data obtained were meager, because breeding activity was usually inhibited in voles kept in confinement. A female live-trapped on July 23, 1951, appeared to be in breeding condition. When trapped two days later, she had a copulatory plug, and 21 days after this she was found with a newborn litter in a trap. A female thought to have given birth to a litter between successive captures on July 20, and July 21, 1951 (on the basis of appearance of genitalia, and reduction in weight from 53 to 46 grams), appeared to have just completed parturition when she was examined on August 10. A female that gave birth to a litter in confinement on May 18, 1954, bred and was released the same day. She was recorded as pregnant in the first week of June, but on June 7 was no longer pregnant. If this pregnancy terminated normally, a gestation of 20 days or less is indicated. Greenwald (1956:221) suggested that in _M. californicus_, oestrus might occur in the period of lactation, because he found recently formed corpora lutea in lactating females. In the course of my field work on _M. ochrogaster_, I obtained precise or approximate dates of successive litters born at intervals of somewhat more than 21 days apart. In different females, intervals of 23, 23, 24, 26, and approximately 27 (between 26 and 28) days were recorded between successive litters. In four other females intervals between litters were known only approximately because one of two records was based on a capture in late pregnancy judged to be within two or three days of parturition. For these females, intervals of 23, 24, 24, and 26 days were recorded. From the trend of these records, it seems that females often became pregnant within a few days after birth of a litter. Pregnancy from post-partum oestrus would seem to be less frequent than pregnancies beginning a few days after birth of the previous litter, and within the period of lactation. NUMBER OF YOUNG PER LITTER Jameson (1947:146) found an average of 3.4 young per litter in 58 litters of _M. ochrogaster_ from northeastern Kansas, mostly from Douglas County. Martin (1956:386) recorded a somewhat lower mean of 3.18 ± 0.24 in 65 litters on the Reservation in 1950, 1951, and 1952. For a total of 82 litters recorded from 1950 through 1956, inclusive, I obtained an average of 3.37 ± .075 young per litter. Several litters that were recorded were excluded from this computation as in each instance there was reason to suspect that they were incomplete. These included instances of females found in traps with young several days old, females that may not have completed parturition when they were released with newborn young, and those litters that might have sustained losses through cannibalism by the mother or her trap-mates.