The subunit structure of hemoglobin from the clam shrimp Cyzicus was investigated. The native protein has a molecular weight of 280,000 as shown by sedimentation equilibrium. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis in the presence of mercaptoethanol resulted in a single band corresponding to a molecular weight of 15,500. A molecular weight of 14,500 was determined by the method of sedimentation equilibrium for the isolated polypeptide chain. Electron microscopy of the protein revealed a pentagonal structure. It was concluded that the native molecule contains 20 polypeptide chains.