Piabucina festae, a Central American stream fish, breathes air frequently, even in air saturated water, however, is not an obligate air breather. Without access to air, it can maintain routine\(\dot V_{O_2 }\) by aquatic respiration down to aPwO2 of about 70 Torr which is its critical O2 tension (PcO2, Fig. 5). Aerial respiration averages 10% of total\(\dot V_{O_2 }\) in air saturated water and 70% in hypoxic water (Fig. 4). At lowPwO2 air breathing is more frequent (Fig. 1), and more O2 is utilized from each air breath (Table 3), and tidal volume may increase (Fig. 7). Vascularized respiratory compartments or cells (Fig. 6), located in the second chamber of the physostomus gas bladder, function for aerial respiration. In ventilation air is gulped and forced through a large pneumatic duct into the gas bladder, excess gas is then released through opercula. Inspiration always precedes expiration and tidal volume is small, keeping gas bladderPO2 low (Table 4). Major differences in the air breathing physiology ofP. festae and other species are its higherPcO2, a low aerial\(\dot V_{O_2 }\) in normoxic water, even though air gulps are frequent, and its pattern of inhalation prior to expiration. The interrelationship and optimization of the three gas bladder functions (buoyancy, sound reception, and air breathing) inP. festae is discussed. Aerial respiration may have evolved secondarily to the gas bladder's function in buoyancy control.