We investigated the ability of the tropical social wasp Polybia occidentalis to recognize local species of scouting-and-recruiting ants by their odor. Wasps in colonies that had not recently experienced Crematogaster ?rochai or Solenopsis geminata did not respond to the odors of these ants. Following contact with these ants, however, some of the individuals responded defensively to the odor of these ant species, showing that wasps can learn the odor of ants. Wasps were better able to learn to respond to the odor of Crematogaster than to Solenopsis. The results of this preliminary field study point to a role for learning in the defensive behavior of social wasps.