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1. The effect of different extracellular alkaline-earth cations (Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺) upon the threshold membrane potential for spike initiation in crayfish axon has been studied by means of intracellular micro-electrodes. This was done at the following pHs of the bathing saline: 6·00, 5·35, 5·00, and 4·65 ± 0·05.

2. At pH 6·00, the four alkaline earths had essentially the same effect upon the threshold membrane potential.

3. At pH 5·35 or lower, it was found that equal concentrations of the alkaline-earth cations did not have the same effect on the threshold potential. The selectivity sequences observed at the different pHs were: pH 5·35, Ca²⁺ > Mg²⁺ > Sr²⁺ Ba²⁺; pH 5·00, Ca²⁺ Ba²⁺ > Sr²⁺ Mg²⁺; pH 4·65, Ba²⁺, Sr²⁺ > Ca²⁺ > Mg²⁺.

4. It is shown that the individual selectivity sequences are predicted rather closely by the equilibrium selectivity theory for alkaline-earth cations.

5. It is concluded that the only difference between excitable cells which show screening and those which bind divalent cations is the net density of ionized, surface acidic groups in the region of the sodium gates.