1. Rates of oxygen consumption for calling, rest and locomotion were measured for three species of treefrogs from the south-east United States: Hyla cinerea (Schneider), H. gratiosa LeConte and H. squirella Bosc. 2. Anaerobic metabolism was slight during calling but large during forced hopping in H. squirella . It is assumed that calling in all three species is primarily aerobic. 3. For all species, oxygen consumption during calling rivalled or exceeded that for locomotion in a closed chamber. Calling apparently is at least as costly as closed-chamber locomotion, if anaerobic contributions to hopping are ignored. 4. The mass-specific costs of producing calls are similar in most species of hylids whose calling energetics have been studied. 5. As in other species of hylids, abdominal trunk muscle masses in male H. squirella and H. gratiosa were large - approximately 10% of the total body mass. 6. Complete sound fields (with measures of variation) were mapped for each species. Each species is an essentially omnidirectional radiator with only a slight flattening of the field behind the head. 7. Efficiencies of sound production (acoustic power/metabolic power) were calculated for each species and compared with values from other frogs, insects and loudspeakers. Efficiencies in hylids vary between 0.8 and 4.9%. 8. Factors that contribute to determining the efficiency of sound production are reviewed. Small animals such as frogs and insects must either use low-efficiency radiators or use high-frequency sounds that do not propagate over great distances. It is possible that low-efficiency transducers maximize the area over which an animal is heard per unit of energy expended.