Present social organization and mating systems result from selective pressures and ecological conditions but also from proximate interactions between individuals. Many studies report on a polygynous mating system with a social group territoriality in commensal populations of Mus musculus domesticus. However, little is known about the social organization of other Mus species living in outdoor conditions, such as the mound-building mouse Mus spicilegus. Comparative studies between M. m. domesticus and M. spicilegus have already shown behavioral differences in female sexual preferences and paternal care. To study agonistic and sociable interactions and gain insight into the social organization and the mating system of M. spicilegus, the present study compared intraspecific dyadic encounters between unfamiliar adults in these two species. Results demonstrated less tolerance between females and between the sexes in M. spicilegus than in M. m. domesticus unfamiliar mice. The consequences of those differences between M. spicilegus and M. m. domesticus on social organization and mating system are discussed. Aggr. Behav. 28:75–84, 2002. © 2002 Wiley-Liss, Inc.