The respiration physiology of Piabucina festae, an air-breathing characin fish in the family Lebiasinidae, has been compared with that of P. panamensis, a closely related non-air-breathing species. The gas bladder of P. festae is highly modified for rapid O₂ absorption through the presence of a specialized region in the posterior chamber. In P. panamensis this organ has no respiratory specializations, but low rates of O₂ absorption do occur. Piabucina festae has a higher metabolic rate than P. panamensis and can maintain its routine $\dot{V}O_{2}$ without air breathing in normoxic water. The capacity for aquatic respiration of P. festae is reduced compared with that of P. panamensis. Piabucina festae has a lower resistance to aquatic hypoxia. It cannot greatly increase its rate of gill ventilation and has a critical oxygen tension (PCO₂) of 70 torr. By contrast, P. panamensis can increase gill ventilation by five times and has a critical PCO₂ of 25 torr. Air-breathing characins occur only in the Neotropic...