Abstract Gap junctions between segments of the crayfish septate axon mediate electronic transmission of impulses propagating along the length of the nerve cord. We simultaneously measured intracellular pH (pH i) and gap junctional conductance (g j) while axons were exposed to saline equilibrated with CO 2, weak acids, and the weak base ammonium chloride. Normal pH i is about 7.1. When pH i is elevated, g j is unaffected. When pH i is reduced, g j declines with an apparent pK of about 6.7 and a Hill coefficient of about 2.7. We also measured effects of pH i on non-junctional conductance (g nj) and on the coupling coefficient, k. Over the pH i range 6.2–8, g nj increases approximately linearly with pH i. Since k is a function of g j and g nj, it reached a maximum at about pH i 7.1, decreasing at higher and lower pH i. The pH i dependence of g j in crayfish septate axon is less steep and has a lower apparent pK than the g j-pH i relation in two vertebrate embryos previously examined. This finding illustrates a difference in gating among analoguos and possibly homologous membrane channels.