Résumé : In contrast with the hemoglobins of the vertebrates, which are almost invariably intra-cellular and tetrameric, the intra- and extracellular hemoglobins of the invertebrates show a wide variety in their molecular size (Mr 16,000 up to 1.7 x 106) and architecture. 1 - 5 With the exception of the hemoglobins of the chironomid larvae, 6 practically all of the extracellular hemoglobins have a high Mr which is probably necessary to avoid elimination from the hemolymph by excretory processes and an excessive osmotic pressure. 4 Despite the heterogeneity of invertebrate hemoglobins, Svedberg and Hedenius 7 suggested that all these pigments are composed of myoglobin-like polypeptide chains of Mr 16,000 containing one heme group and able to bind oxygen reversibly. Polypeptide chains or fragments of much longer chains having these characteristics (Mr 16,000; one heme) were defined by Vinogradov 5 as “heme-binding domains”. Based in the number of domains and subunits in the native molecule, the invertebrate extracellular hemoglobins can be classified into four groups: Single domain, single-subunit hemoglobins consisting of a single polypeptide chain, containing one heme group and having an Mr 16,000 (Chironomus) Single-domain, multisubunit hemoglobins consisting of aggregates of monomeric subunits, some of which are disulfide bonded (Annelida) Two-domain, multisubunit hemoglobins consisting of aggregates of dimeric polypeptide chains (Mr 30,000 to 40,000), each containing two heme-binding domains (Arthropoda, Nematodd) Multidomain, multisubunit hemoglobins consisting of two or more polypeptide chains each comprising from eight to twenty heme-binding domains (Arthropoda, Mollusca)