The subunit structure of earthworm erythrocruorin was studied. The native protein was found to have a sedimentation coefficient of 61.1 S and a molecular weight, as determined by sedimentation equilibrium, of 3.84 × 106. Dissociation of the 60 S molecule was observed at acid and basic pH. Calcium ion was found to prevent dissociation in the basic range. Dissociated species of 10.1, 3.5 and 2.3 S were isolated and their molecular weights determined to be 163 × 103, 41 × 103 and 22 × 103, respectively. On a molecular weight basis, the native molecule consists of twenty-four 10 S subunits, each of which is composed of four 3.5 S units, which are further divided into two 2.3 S units. The 2.3 S species contains a single heme per molecule. A model of the 60 S molecule, where the twenty-four 10 S subunits are arranged into four concentric layers of six subunits each, is proposed. Support for this arrangement is provided by the excellent agreement between the sedimentation coefficient of a 1 24th unit calculated by the application of Kirkwood's (1954) theory with the experimentally determined sedimentation coefficient of the 10 S subunit. An 88 S aggregate observed at the transition pH of acid dissociation is attributed to a side-by-side association dimer of the 60 S molecule.