The call-notes of the Greater Rhea chick were classified according to their acoustical structure and behavioral context. Both rate of vocalization and tonal quality of call-notes were found to deteriorate as a function function of maturation. Call-note production was completely eliminated by 7 weeks of age. Greater Rhea adults are silent except for such relatively infrequent vocalizations as "hissing" that sometimes accompanies agonistic behavior and a disyllabic "grunt" that only the male produces as part of a courtship display. Tracheal growth and a marked reduction in the intrusion of the internal tympanic membranes into the bronchial passages could contribute to the deterioration of vocalization. The use of vocalization as the primary channel of communication is replaced by visual display in the adults. These modalities are best suited to the ecological contingencies facing the chick and adult, respectively.