The 3-5 year cyclical fluctuations in populations of many vole and lemming species have perplexed ecologists for many years. Numerous hypotheses have been proposed to explain microtine rodent cycles, including various aspects of social behavior. Microtine rodents commonly form kin groups composed of related females. Charnov and Finerty (1980) proposed that the formation and breakup of kin groups could, in part, explain the rates of population increase and decline associated with cycles. My experiment sought to determine if kin groups provided population-level benefits in gray-tailed voles, Microtus canicaudus. I compared unmanipulated populations with populations in which kin-structuring was experimentally disrupted to determine if kin groups affected population growth rates and size, reproduction, pregnancy and lactation rates, and recruitment, movement and survival of juveniles. I monitored demography and reproductive behavior in eight 0.2 ha experimental enclosures during a summer breeding season. I found no differences in demographic or female reproductive parameters between control and treatment enclosures, with the exception of a delayed time to first pregnancy for females introduced into the treatment enclosures. In addition, I found no differences in the time to sexual maturation or dispersal movements of juvenile males between control and treatment enclosures. I conclude that disrupting the formation of kin groups does not adversely affect demographic or reproductive parameters at the population-level in gray-tailed voles, and suggest that the contribution of kin groups to social behaviors that may affect population regulation is probably quite small. / Graduation date: 1998