Examples of the two main classes of circulating respiratory pigments (heme pigments and hemocyanins) have been found in four typical members of the deep-sea hydrothermal vent communities. The studies now in progress concern the hemoglobin of Calyptogena ; the erythrocruorins of Alvinella and Riftia ; and the hemocyanin of Bythograea . This paper summarizes the available data of the molecular structure of these pigments and the functional properties allowing some of them to fix and transport oxygen, sulphide and carbon dioxide. The erythrocruorins of Riftia are the most conspicuous of these respiratory proteins since they seem to be able to carry oxygen and sulphide simultaneously, these substances being fixed at two different sites of the pigment molecule.