In the American lobster, Homarus americanus, oxygen is transported by a hemocyanin that is composed 2 x 6 subunits. N-terminal sequencing show the presence of three distinct subunit types (alpha, beta and gamma). We cloned the cDNA of one of these subunits that belong to the alpha -type. It encodes a hemocyanin subunit of 654 amino acids with a molecular mass of 84.8 kDa, which is synthesized in the hepatopancreas. Phylogenetic analyses of the crustacean hemocyanin sequences show two well-separated clades, which correspond to the alpha and gamma -type subunits. Sequences of P-type subunits are still unknown. The gamma -sequences have evolved about 15% faster than the a-subunits, consistent with the proposed conservative function of the latter, Under the assumption of a molecular clock we calculated that alpha- and gamma -subunits split about 214 +/- 14 million years ago, suggesting their divergence only in the decapod Crustacea.