I. The vocal repertoire of bullfrogs maintained in laboratory terrariums in a semi-natural environment consisted of a distinct set of stereotyped calls. Each call could be readily identified with specific patterns of behavioral activity, thus indicating the significance represented by each of these sounds. 2. Mating calls were made only by males. The call by an adult male often evoked calling from other male bullfrogs; the call by a juvenile male did not evoke calling. 3. Territorial calls consisted of three distinct types: (i) one type made only by males, (ii) a second type made only by females, and (iii) a third type made by both males and females. These calls seemed to alert other bullfrogs that a territory was occupied and was being trespassed. 4. Release calls were made by both sexes. When an unreceptive female or a male was clasped by another male, the clasped animal first usually struggled and then emitted this call, whereupon it was released. 5. Warning calls were made by both sexes. This call was often made prior to a leap into or out of the terrarium ponds. It also was sometimes made by a bullfrog shortly after having produced one of the other calls in his vocal repertoire. 6. Distress calls, made by both sexes, could be evoked by repeated application of a noxious stimulus. This high-pitched call was always made with the mouth wide open. 7. The bullfrog's peripheral auditory system is well suited to detect and encode the calls in his vocal repertoire.