The Asian swamp eel (Monopterus albus, Zuiew 1793) is a facultative air-breathing fish with reduced gills. Previous studies have shown that gas exchange seems to occur across the epithelium of the buccopharyngeal cavity, the esophagus and the integument, resulting in substantial diffusion limitations that must be compensat- ed by adaptations in others steps of the O2 transport system to secure adequate O2 delivery to the respiring tissues. We therefore investigated O2 binding properties of whole blood, stripped hemoglobin (Hb), two major isoHb components and the myoglobin (Mb) from M. albus. Whole blood was sampled using indwelling catheters for blood gas analysis and determination of O2 equilibrium curves. Hb was purified to assess the effects of endog- enous allosteric effectors, and Mb was isolated from heart and skeletal muscle to determine its O2 binding prop- erties. The blood of M. albus has a high O2 carrying capacity [hematocrit (Hct) of 42.4 ± 4.5%] and binds O2 with an unusually high affinity (P50 = 2.8 ± 0.4 mmHg at 27 °C and pH 7.7), correlating with insensitivity of the Hb to the anionic allosteric effectors that normally decrease Hb-O2 affinity. In addition, Mb is present at high concen- trations in both heart and muscle (5.16 ± 0.99 and 1.08 ± 0.19 mg ∙ g wet tissue-1, respectively). We suggest that the high Hct and high blood O2 affinity serve to overcome the low diffusion capacity in the relatively ineffi- cient respiratory surfaces, while high Hct and Mb concentration aid in increasing the O2 flux from the blood to the muscles.