Darters (Anhingidae) are among the most territorial Pelecaniformes, but most of the observed aggression is between males and limited primarily to the breeding season. We observed three instances of agonistic interactions between two female Anhingas (Anhinga anhinga) foraging in a pond of an urban park in southeastern Brazil. A foraging resident female chased another female as soon as it caught sight of the latter approaching, which caused the intruder to dive and retreat. The resident female vocalized toward the site where the intruder disappeared while still in the water. The resident then vocalized from a perch from time to time toward the pond while drying its feathers. We played back its recorded call and the bird vocalized toward us suggesting this vocalization was territorial. Agonistic behavior at foraging sites merits further observation to learn whether it is restricted to particular individuals and/or periods.