Females in confinement were attentive to young, and, soon after parturition, licked them clean and huddled over them protectively. Ordinarily, the newborn young soon attached to a teat, and spent a large part of its time attached during its early development. Females found in live-traps with their litters of young less than a day old, often had some or all of the young clinging to their teats. Females with newborn litters, when released from live-traps, always left without attempting to retrieve any young that were unattached. Such young usually were permanently deserted, but in some instances disappeared within an hour or less, perhaps rescued by the female returning for them. Females with newborn young were made far more aggressive than most other voles by their tendency to protect their young from possible danger. In captivity such females usually took the offensive in attacking or rebuffing any other voles confined with them. Post-partum females obviously in oestrus were prevented from being fully receptive by their hostility toward males whose presence might endanger the young. Such a female has been seen to turn on a pursuing male and attack him viciously, several times within a few minutes, before copulation occurred. In captivity, at least, such attacks would soon discourage a male so that unless he was exceptionally active sexually, mating was prevented. Cannibalism, involving destruction of the newborn, is probably an important factor in the population dynamics of the prairie vole. Only a small percentage of the young known to have been born on an area ever survived to be live-trapped; this small percentage was indirect evidence of decimating losses in the young. Under unfavorable conditions each of several females killed and ate her own litter, but the degree of provocation varied greatly among individuals. Females that gave birth to young in live-traps occasionally ate one or more of their newborn young, as evidenced by discarded remnants. Perhaps other instances passed unnoticed because no remnants were found. That need for food or moisture as well as psychological stress often motivated such cannibalism was suggested by the fact that surviving litter mates might be accepted and cared for by a female that had already eaten one or more of her young. Although cannibalism is most likely to occur within a few hours after birth of the young, they may be killed and eaten at any stage of development. One female that had probably eaten one or more of her litter, soon after parturition, nursed the two survivors. When these were two weeks old, all were "pastured out" in a wire mesh cage in tall brome grass. When the supply of grass had become scarce (though some was still available), the female killed and partly ate both her remaining young.