At many facilities, limitations of the physical environment have reduced the opportunity for captive chimpanzees to live in large, naturalistic social groups. Convex mirrors used to increase visual access of neighboring groups may improve the social environment. This was tested in a study of 28 chimpanzees (Pan troglodytes) group-housed in conventional indoor/outdoor runs. A total of 47.8 hours of behavioral observations were conducted and comparisons made across three conditions: no mirror present, a mirror present with visual access to neighboring conspecifics, or a mirror present with visual access to the neighbors' empty run. When the mirror gave subjects visual access to neighboring animals, facial expressions, sexual, and agonistic behaviors increased, whereas affiliative behavior decreased compared with when no mirror was present. When the mirror gave subjects visual access to a neighbors' empty run, facial expressions and sexual behavior increased compared with when no mirror was present. When the mirror gave subjects visual access to a neighbor's empty run, agonism decreased compared with when a mirror gave subjects visual access to neighboring animals. When subjects had visual access to neighbors, they used the mirror 30% of the total data points; while they had visual access to the neighbors' empty run, they looked during 24% of the total data points. Juveniles' use of the mirror increased over time while adults' use remained stable. Adult males used the mirror less than did the other subjects. These findings indicate that a mirror allowing visual access to neighboring conspecifics has potential as an enrichment device that affects social behavior.