The relationship between whole blood-oxygen affinity (P(50)) and pH-dependent binding (i.e., cooperativity and the Bohr [ Phi ] and Root effects) was examined statistically under standardized conditions (10.0 degrees Celsius) in four unrelated cold-temperate marine fishes that differ widely in their swimming performance and their expected responses to hypoxia: cod (Gadus morhua), herring (Clupea harengus), mackerel (Scomber scombrus), and plaice (Pleuronectes platessa). An unexpected difference in blood-oxygen affinity was found (herring>plaice>mackerel>cod), and this was independent of both swimming performance and the predicted low O(2) response of each species. The ecotype of the four marine species was also unrelated to pH-dependent binding because no difference in the Bohr effect was apparent ( Phi varied insignificantly from -0.90 to -1.06), and differences in the magnitude of the cooperative binding reaction were associated only with the presence of the Root effect. Although several reviews propose a generalized link between blood-oxygen affinity and pH-dependent binding, our results advise against overestimating the adaptive functional properties of hemoglobin across unrelated species.