Structural analysis of the hemoglobin (Hb) system of Delphinus delphis revealed a high globin multiplicity: HPLC–electrospray ionization-mass spectrometry (ESI-MS) analysis evidenced three major β (β1 16,022 Da, β2 16,036 Da, β 3 16,036 Da, labeled according to their progressive elution times) and two major a globins (α1 15,345 Da, α2 15,329 Da). ESI-tandem mass and nucleotide sequence analyses showed that b2 globin differs from b1 for the substitution Val126 → Leu, while β3 globin differs from β2 for the isobaric substitution Lys65 → Gln. The α2 globin differs from the a1 for the substitution Ser15 → Ala. Anion-exchange chromatography allowed the separation of two Hb fractions and HPLC–ESI-MS analysis revealed that the fraction with higher pI (HbI) contained β1, β2 and both the a globins, and the fraction with lower pI (HbII) contained β3 and both the a globins. Both D. delphis Hb fractions displayed a lower intrinsic oxygen affinity, a decreased effect of 2,3-BPG and a reduced cooperativity with respect to human HbA0, with HbII showing the more pronounced differences. With respect to HbA0, either the substitution Proβ5 → Gly or the Proβ5 → Ala is present in all the cetacean β globins sequenced so far, and it has been hypothesized that position 5 of β globins may have a role in the interaction with 2,3-BPG. Regarding the particularly lowered cooperativity of HbII, it is interesting to observe that the variant human HbA, characterized by the substitution Lysβ65 → Gln (HbJ-Cairo) has a decreased cooperativity with respect to HbA₀.