Lampreys belong to the class of Cyclostomata; practically no evolution of these Vertebrates can be noted since Paleozoïc times; lampreys thus appear as a choice material for studying several problems in the field of biochemical evolution. Several monomeric haemoglobins can be characterized in the erythrocytes of the sea lamprey (Petromyzon marinus). The major constituent was isolated by chromatography, and submitted to tryptic digestion; soluble tryptic peptides were separated by gel filtration into 5 fractions; the peptides of each fraction were isolated either by Dowex-50 chromatography or by HPLC; the insoluble core was oxidized and submitted to HPLC fractionation. The primary structure of the whole chain and of the purified tryptic peptides was determined using automatic sequencing; alignment of the peptides was achieved by homology with the previously established covalent structure of the globin of Lampetra fluviatilis. The sequence we established confirms the crystallographic data of Hendrickson and Love. Globin/haem contacts are discussed; a tentative explanation of the absence of tetramerization can be proposed after comparison with the aminoacid residues involved in alpha 1 beta 1 and alpha 1 beta 2 contacts. Petromyzon globin differs at three locations (Thr/Ser3, Leu/Met58, Thr/Ser60) from Lampetra fluviatilis globin. The monomeric chain of another Cyclostomata Myxine glutinosa, differs more considerably (88 residues). Our results corroborate recent paleontologic data which favour the separation of lampreys from hagfishes; Cyclostomata cannot be considered as a monophylic group. Finally, there is a closer relation between lamprey globin and alpha chains than between this monomeric globin and beta chains, and furthermore apomyoglobins of higher vertebrates.