Ants learn the odors of members of their colony early in postnatal life, but their ability to learn to recognize noncolony conspecifics and heterospecifics has never been explored. We used a habituation-discrimination paradigm to assess individual recognition in adult Formicine ants, Cataglyphis niger. Pairs of workers from different colonies were placed together for repeated trials, and their ability to discriminate the ant that they encountered from another familiar or unfamiliar ant was observed. Some ants were isolated between encounters, and others were returned to their home colonies. Our results suggest for the first time in ants that C. niger adults learn about individual ants that they have encountered and recognize them in subsequent encounters. Ants are less aggressive toward non-nestmates after they are familiar with one another, but they are aggressive again when they encounter an unfamiliar individual. Learning about non-nestmates does not interfere with an ant's memory of members from its own colony.