The complete amino acid sequence of the Rapana thomasiana hemocyanin N-terminal functional unit Rta was determined by direct sequencing and matrix-assisted laser desorption ionization mass spectrometry of the protein and peptides obtained by cleavage with EndoLysC proteinase, TPCK-trypsin and cyanogen bromide. The single polypeptide chain consists of 407 residues. This is the first report on the primary structure of a dioxygen-binding unit from a marine gastropod hemocyanin and of an N-terminal domain from a molluscan dioxygen carrier. Comparison with the sequences of other molluscan hemocyanin functional units shows an average identity of 48 +/- 5 %. Inspection of the Rta sequence revealed residues 27 and 250 as carbohydrate attachment sites. Conclusions about the molecular evolution of the molluscan hemocyanin dioxygen-binding functional units are made.