The air-breathing and economically important tropical fish Pangasianodon hypophthalmus thrives in severely hypoxic (down to <3mmHg) and warm waters. To increase our understanding of its respiratory physiology, we characterized the oxygen-binding properties of isolated hemoglobin (Hb) and whole blood. Pangasius has a high blood O2 affinity (P50=9.2 mmHg) compared to similarly active fish species (trout P50=24, salmon P50=21, tuna P50=16), but comparable to that of other, more sluggish air-breathing fish (P50 7-11 mmHg). The high oxygen affinity of Pangasius blood stems from low Cl- sensitivity, rendering O2 binding strongly temperature sensitive and reducing blood oxygen affinity at higher temperatures (?P50/?T=0.44mmHg/oC). Pangasius has only one distinct iso-Hb and cannot therefore change isoform expression pattern at elevated temperatures. In addition the red cells appear to have very little response to adrenergic stimulation. O2 transport and in vivo O2 saturation in Pangasius therefore is unusually susceptible to increasing temperatures impacting in vivo O2 saturation.