Coelomic cells from the sea cucumber Caudina (Molpadia) arenicola contain four major globins, A, B, C and D. The hemoglobins from this organism show unusual ligand-linked dissociation properties. The complete amino acid sequence of the D globin has been established. It is N-acetylated, consists of 158 residues and has a 10 amino acid N-terminal extension similar to that found in some other invertebrate globins. The C. arenicola D globin has an equal sequence identity (28%) with both alpha and beta human globins and as anticipated, is more closely related to these vertebrate proteins than are molluscan globins. The C. arenicola D globin shows a 59% identity with the globin I from the sea cucumber Paracaudina chilensis. The availability of the C. arenicola D globin sequence will aid the X-ray analysis of this protein and facilitate an understanding of the changes in subunit interactions that occur with cooperative ligand binding.