In an investigation of the consequences of the hypothesis that the primary cause of the population cycles of the vole (Microtus agrestis) is intraspecific strife, two experimental vole populations were studied for a total of 18 months. The populations were kept in large, cement, open-air cages, and weekly censuses were made of the number of animals present. One of the populations initially contained four times as many animals as the other. With the onset of breeding both populations increased in size, but the growth rate of the larger was less than that of the smaller, so that the populations became much more nearly equal in size. The initial size of the larger population was considerably greater in its second than its first year, yet the maximum size in the second year was only slightly greater than the first year. The larger population had a shorter breeding season, and females born in it were less fertile than the corresponding females of the smaller group. Within each population, the average survival rate of the infants and juveniles was less than that of the adults, but between the populations there was no marked difference in the survival rate of any of their age classes. The average infant survival rate was considerably less in the second year of the growth of the larger population than in its first. Voles were seen chasing and fighting each other in both populations, but this aggressive activity and the consequent wounding were more frequent in the larger group. It is suggested that the strife between the members of each population was limiting their growth, but that this restraint was more severe in the larger than the smaller group.