In cross-fostering the young of one species are reared by adults of another, as in the classical ethological studies of imprinting and song-learning. In our laboratory, infant chimpanzees were reared under human conditions that included two-way communication in American Sign Language (ASL) the gestural language of the deaf in North America. Here we describe the cross-fostering conditions of this laboratory and the contrast with operant conditioning.We also review the uses and the shapes of the signs in the vocabularies of five cross-fostered chimpanzees, tests demonstrating that the cross-fostered chimpanzees could use signs of ASL to communicate conceptual information to human observers whose only source of information was the signs of the chimpanzees, early development of inflections that resemble the early inflections of deaf human children, evidence based on errors and on inflections for duality of patterning, the continued use of sign language among the chimpanzees even when deprived of human input and the acquisition of signs by the infant Loulis from the cross-fostered chimpanzees.