The subunit structure of erythrocruorin from the crustacean Caenestheria inopinata was studied. The native protein was found to have a sedimentation coefficient of 12.0 S and a molecular weight, as determined by sedimentation equilibrium, of 302 000. Iron and heme determinations gave 0.346 and 3.98% corresponding to minimal molecular weights of 16 100 and 15 500, respectively. Sodium dodecyl sulfatepolyacrylamide gel electrophoresis gave one band with mobility corresponding to a molecular weight of 30000. The molecular weight of the polypeptide chain was determined to be 30 500 by sedimentation equilibrium in 6 M guanidine hydrochloride and 0.1 M 2-mercaptoethanol. Dissociation of the 12S molecule was observed at acidic and alkaline pH. A dissociation species of 2.7 S was isolated and its molecular weight determined to be 28 000 by sedimentation equilibrium. On a molecular weight basis, the native molecule is composed of ten 2.7S subunits, each of which consists of a single polypeptide chain carrying two hemes. We propose a model for the molecule composed of ten spheres, each representing a 2.7S subunit, arranged in two layers stacked in an eclipsed orientation, the five spheres of each layer occupying the vertices of a regular pentagon. Support for this arrangement is provided by a comparison of projections of the model with molecular profiles seen in the electron microscope.