Interspecific competition commonly selects for divergence in ecology, morphology, or physiology, but direct observation of interspecific competition under natural conditions is difficult. Herbivorous white-tailed prairie dogs (Cynomys leucurus) employ an unusual strategy to reduce interspecific competition: they kill, but do not consume, herbivorous Wyoming ground squirrels (Urocitellus elegans) encountered in the prairie dog territories. Results from a 6-year study in Colorado USA revealed that interspecific killing (IK) of ground squirrels by prairie dogs was common, involving 47 different killers; 19 prairie dogs were serial killers in the same or consecutive years, and 30% of female prairie dogs killed at least one ground squirrel over their lifetimes. Females that killed ground squirrels had significantly higher annual and lifetime fitness than nonkillers, probably because of decreased interspecific competition for vegetation. Our results document the first case of IK among herbivorous mammals in the wild, and show that IK enhances fitness for animals living under natural conditions.