This study reports on the aggressive behavior of the gall-forming aphidSmynthurodes betaeWest. Videotaped behavior and experiments demonstrated that first instar offspring of the fundatrix protect their galling sites against conspecific competitors (the larger individuals win), as well as against first instar competitors of the sympatric galling aphidForda riccobonii(Steph.). In mature galls, adult aphids also protect their gall and their offspring from conspecific invaders and natural enemies. The fighting behavior of the adult aphid is unique: the aphid seizes the intruder while lying on its back, enabling use of all six legs and the mouthparts. This is the first documentation of aggressive defensive behavior by adult aphids. Although the aggressive behavior of the adult aphid protects the gall and offspring, no distinct parental care behavior was observed. The territoriality and aggression are highly adaptive because of the limited number of galling sites. Under natural conditions, removal of the adult aphids from the galls resulted in significantly increased invasion by conspecific intruders. The results indicate that the role of interspecific competition and natural enemies in shaping adult behavior is rather limited. The open galls and the unique life-cycle ofS. betaeencourage and prolongintraspecific competition and facilitate confrontations between adults and invading nymphs. The ability of the adult owner of the gall to prevent invasions of nymphs has promoted the evolution of maternal defense in this aphid. This study supports the hypothesis that galls, as high value resources that combine food and shelter, are often associated with defensive and social behavior.