A communal nesting group, formed by additions to a monogamous pair breeding unit, is the basic year-round social unit of the prairie vole, Microtus ochrogaster. Before communal groups form, presumed monogamous pairs display traits normally associated with monogamy including cohabitation of a common nest, a shared home range, and dissolution of the pair primarily by mortality of one or both members. When juvenile nest mortality was reduced during late autumn-winter, extended-family groups formed. There was no difference in natal dispersal between summer and autumn-winter. Approximately 70% of the animals added to monogamous-pair breeding units were philopatric young. Unrelated adults (30% of the additions) joined extended-family groups in equal proportions in summer and late autumn-winter. Approximately half of the unrelated adults were reproductive when they joined an extended family group. During periods of winter reproduction most winter communal groups included at least one reproductive male and female, approximately half included two or more reproductive males and females (half of these were unrelated adults).