A comparison was made of the intra- and interspecific aggressive behavior of the prairie vole, Microtus ochrogaster , and the meadow vole, M. pennsylvanicus . M. ochrogaster was found to display little aggressive behavior towards others of its own species; when confined together, individuals almost always huddled and groomed each other within a few minutes' time. M. pennsylvanicus was intolerant of other individuals of its own species; individuals attempted to avoid each other whenever possible and fought when forced into an encounter. M. ochrogaster , although it displayed less aggressive behavior in general, appeared to be dominant over M. pennsylvanicus . There was less interspecific aggressive behavior between M. ochrogaster and M. pennsylvanicus than intra-specific aggressive behavior in M. pennsylvanicus .