Riftia pachyptila vascular blood and coelomic fluid contain two hemoglobin molecules that differ in their distribution and physical properties. The present study of the two isolated hemoglobins shows that both have an extremely high affinity for oxygen, but differ in their oxygenation characteristics. FI, the larger molecular weight (Mr) fraction (1,700,000), has a lower oxygen affinity, a well defined pH Bohr effect, and high cooperativity of oxygen binding. FII, the lower Mr fraction (400,000) has a higher oxygen affinity, no pH Bohr effect, and reduced cooperativity of oxygen binding. Both hemoglobins show marked effects of temperature on oxygen binding, and no effect of heme concentration or the presence of sulfide on oxygen affinity. The differences in the oxygenation properties and distribution of the two hemoglobins in the body fluids of Riftia pachyptila may allow them to play different roles in oxygen transport and storage for the animal which lives in the variable environment of the hydrothermal vents.