Abstract: The behavior of nine lowland gorillas (Gorilla gorilla gorilla) living in three social groups at Zoo Atlanta was compared in an indoor holding area versus an outdoor exhibit. Focal animal data were collected for each animal during 15 min observation sessions, alternating between indoors and outdoors. A variety of solitary and social behaviors differed in the two conditions. All individual and social behaviors that showed a difference, except eating, occurred more indoors than outdoors. These included aggressive displays, reclining, self manipulation, and social examination of others. Additionally, the gorillas spent more time closer together in the indoor condition. A variety of other behaviors measured did not change between the two environments. There was a clear effect on behavior of the different housing conditions in which the gorillas were kept. It is suggested that the differences in aggressive behavior may be related to environmental complexity. It is further suggested that zoos should be aware that differences in behavior reported by caretaking staff, researchers and visitors may be a reflection of the differing environmental circumstances in which the animals are observed.