This anatomical and experimental study has revealed that the banded knife fish Gymnotus carapo is capable of air breathing by means of the highly specialized, enlarged, and vascularized posterior chamber of the gas bladder. Air ventilation is accomplished by the action of a uniquely specialized esophageal force pump. Cineradiography has shown that air ventilation proceeds in a consistent sequence, with an inhalation followed by two or more exhalations. It is postulated that prior inhalation raises the pressure within the gas bladder temporarily as a preliminary step to passive exhalation of air by hydrostatic pressure via a relatively narrow and long pneumatic duct. At temperatures of 29-31 C Gymnotus extracts 1.63, 1.12, and 0.76 mmol O₂ h⁻¹ kg⁻¹ from air (Ṁao2) when it is in air-equilibrated water at 47%, 72%, and 95%, respectively, of air at STP. The amounts of oxygen extracted from water (Ṁwo2) are, respectively, 0.56, 1.03, and 1.33 mmol O₂ h⁻¹ kg⁻¹. Total Ṁo2 between 95% and 47% normoxia in water is 2.23 ± 0.17 mmol O₂ h⁻¹ kg⁻¹. When Po2 of water is higher than 200 torr the fish can rely on water breathing, but when Po2 is below 60 torr it relies on air. When Gymnotus has no access to air, it is able to lower its total O₂ consumption to 69% of the control Mo2 at a temperature of 29 C. Aquatic respiration increases with higher O₂ levels in water, lower temperatures, and when access to air is either hindered or prohibited, while aerial respiration increases under the opposite conditions.