Oxygenation function of annelid erythrocruorin (Er) is affected by Mg and Ca concentration in the blood. Four classes of responces may be encountered in different species: 1) Mg=Ca (equal effects), 2) Mg>Ca, 3) Mg<Ca and 4) no effect. In the marine polychaete Arenicola marina, at physiological pH and 20°C, Mg and Ca exerted almost equivalent effects in increasing oxygen affinity in the range of 1–200 mM. As measured from the slope of Δlog P_1/2 /Δlog [Cation] the effect of Mg was larger than that of Ca at the physiological concentration of respective ion (55 mM Mg; 10 mM Ca). The n_1/2 value was similar in the presence of both cations (pH 7.0) or higher for Mg (pH 7.6). In the terrestrial oligochaete Lumbricus terrestris, at the same condition, Ca was more effective than Mg, in raising oxygen affinity at both pHs, also at the physiological concentration (2–4 mM Mg; 8 mM Ca), and n_1/2 was similar for Mg and Ca (pH 7.0) or higher for Ca (pH 7.6). The Bohr factor, -[Δlog P_1/2 /ΔpH], is maintained its maximum value within the span of the physiological concentration of Mg in Arenicola. In Lumbricus, Ca can contribute to the increase of the Bohr factor at the physiological concentration, but Mg cannot contribute to it. These results reveal that Arenicola and Lumbricus belong to classes 1) and 3), respectively, and that the oxygenation function of both Ers may be controlled by effective utilization of the more dominant of the divalent cations Mg and Ca.