I evaluated the effects of age and experience raising young on behaviors associated with defending maternal territories in primiparous yearling, primiparous adult, and multiparous adult Belding's ground squirrels (Urocitellus beldingi). Intensities of vigilant and aggressive behavior were lowest among primiparous yearlings, intermediate among primiparous adults, and highest among multiparous adults, with differences being greatest during gestation and lactation and declining after the 1st emergence of young from the natal burrow. These differences suggest that both age and experience raising young improve territorial behaviors in this species. I also evaluated responses of female U. beldingi to a nonthreatening (caged) conspecific intruder placed in the maternal territory. Investigation of the intruder and aggression directed at the intruder were relatively low among multiparous females throughout the reproductive period, suggesting that these females were able to discern that the intruder did not pose a threat. Among primiparous adults, investigatory and aggressive responses toward the intruder were high during gestation but declined thereafter, suggesting that as the reproductive period progressed these females were better able to discriminate between a threatening and a nonthreatening stimulus. Among primiparous yearlings, investigatory and aggressive behaviors were low during gestation, and females spent about 30% of the duration of tests watching the intruder without overt vigilance. Investigatory and aggressive behaviors increased during lactation and declined to low levels after emergence of young from the natal burrow, suggesting that after an initial hesitancy to respond to the intruder, primiparous yearlings reacted to the intruder as a threat, but by the end of the reproductive period may have been able to determine that the caged intruder was not threatening. Overall, results of this study are consistent with the idea that both age and experience raising young influence maternal territorial behaviors and discrimination abilities in female U. beldingi.