The hemocyanins from molluscs and from arthropods differ in the size of their polypeptide chains. A variety of physical techniques including sodium dodecyl sulfate polyacrylamide gel electrophoresis and column chromatography in sodium dodecyl sulfate and guanidine HCl indicate that the polypeptide chain of mollusc hemocyanin has a molecular weight of 290,000. These results were corroborated by quantitative end group analyses. Several experiments designed to count the number of tryptophan and methionine-containing peptides in the hemocyanin from the whelk Busycon canaliculatum indicate that sequence homology within the polypeptide chain of the mollusc hemocyanins accounts for their large size. Digestion of the native protein with subtilisin produces a 50,000-dalton fragment in high yield which corresponds to one binding site for oxygen. On the other hand, the polypeptide chain molecular weight of lobster hemocyanin is 76,000 to 78,000 and this seems to be a general property of all arthropod hemocyanins. The pigment from lobster consists of two very similar polypeptide chains which are not present in equal amount. Analysis of the cysteine-containing and of the tryptophan-containing tryptic peptides confirms the value of the molecular weight. However, separation of fragments which contain methionine indicates that there is sequence homology withing the polypeptide chain of this protein. It is concluded that the mollusc and arthropod hemocyanins have little structural similarity.