1. Myxicola giant axons were examined under voltage clamp in solutions with [Ca2+] ranging from 2 to 200 mM. 2. The magnitude of the shifts in the Na and K conductance-voltage curves, as well as membrane rate constants, caused by changes IN [Ca22+] were comparable and consistent with a model in which there exists a negative surface charge density of -0.013 charges/A-2, but there is no significant binding of divalent cations to the external membrane surface. 3. There was no effect of equimolar substitution of magnesium for Ca on the positions of the conductance-voltage curves along the voltage axis, consistent with the above conclusion. 4. Ca2+ has no effect on the shape of the steady-state Na inactivation relation but simply causes a translation along the voltage axis. 5. It is concluded that a simple surface charge model in which divalent cations do not bind is adequate for Myxicola axons, and that there is no evidence to support a specific involvement of Ca with gating processes.