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.