Artemia is unusual in having extracellular hemoglobins of Mr 260,000 comprising two globin chains (Mr 130,000), each of which is a polymer of eight covalently linked domains of about Mr 16,000. The amino acid sequence of one of these domains (E1) has been determined. It has 147 residues and Mr of 17,574 including heme. Sequence alignment revealed 19.0% identity with sperm whale myoglobin, whereas other vertebrate and invertebrate globins had between 13 and 24% identity. However, a much higher percentage of residues has a similar side chain character, suggesting that the domain E1 is very similar to other globins in showing the myoglobin fold. Template model building based on the known three-dimensional structure of myoglobin further supports this conclusion. Conversely, the differences between E1 and other globins are believed to reflect differences in the packing of the domains, first in a covalent polymeric subunit containing eight hemes and subsequently by association of two of these subunits as dimers. These findings provide further evidence for the versatility of the myoglobin fold.