In order to document the effect of temperature and pH on O2- and CO2- transport capacities, we have studied the temperature dependency of hemoglobin (Hb) O2 binding and its allosteric regulation in the striped catfish, Pangasianodon hypophthalmus. We show that Hb of P. hypophthalmus binds O2 with a high affinity and that O2 binding is only slightly affected by pH (Fig. 1a) but strongly affected by temperature (Fig. 1b). Fish erythrocytes contain co-factors, including Cl- and ATP that bind to Hb reducing its O2 affinity. O2- binding, which is exothermic, is coupled to an endothermic release of these allosteric co-factors, countering this O2 binding exothermy. By measuring the O2-affinity at several temperatures, the enthalpy of O2- binding (?Happ) in the presence or absence of co-factors can be calculated. We show that P. hypophthalmus Hb-O2 binding is insensitive to Cl- shown by the superimposed ?Happ?values (Fig. 2). This lowers the base-line from where allosteric regulation can occur and explains the high O2 affinity of P. hypophthalmus blood. This lack of oxygenationlinked endothermic Cl- binding also results in high temperature sensitivity of O2-binding (higher temperature lower O2 affinity). This study provides key information in understanding the O2 transport of pangasius blood and a framework for further investigations of the respiratory physiology of this species. These results imply that in a warmer future world pangasius will need higher environmental PO2 to achieve similar O2 saturations to today the fish of today.