Territoriality is a fundamental social interaction. We have developed a new bacterial model to study intraspecies territoriality. This is the interaction of swarming colonies of the urinary tract pathogen Proteus mirabilis. Two swarms of a single isolate will merge with each other but independent isolates form boundaries between swarms. We have identified the genetic basis of this phenomenon. At its base, the mechanism is that of self-nonself discrimination. This is a new and emerging area in sociomicrobiology.