Abstract: The oxygen binding properties of the hemoglobins from two toads, Bufo bufo and Bufo viridis, have been investigated as a function of protons, chloride ions, organic phosphates and temperature. Electrophoretic analysis of the hemolysates showed the presence of a main hemoglobin in each of the two species. We found that the hemoglobin from Bufo bufo shows at 20°C a slight Bohr effect which tends to increase in the presence of the different allosteric effectors (chloride ions and ATP). At 37°C, the effect of protons is completely abolished in all experimental conditions. The Bufo viridis Hb presents a Bohr effect slightly more pronounced (doubled with respect to B. bufo) which increases only with the simultaneous presence of modulators, both at 20 and at 37°C. Moreover the overall heats of oxygenation (expressed by δH values) result in the two amphibian hemoglobins much less exothermic than that of the human hemoglobin and in the case of Bufo viridis completely independent by organic phosphate (DPG). These particular features are very interesting because the two hemoglobins seem well adapted to the different habitats and physiological needs characterizing the two toads.