Experiments were carried out on the skeletal muscle fibres of the cricket Acheta domesticus using conventional microelectrode methods. Both potassium and ammonium ions depolarized the fibres but with a considerable divergance from the slope of 58 mV. Calcium ions hyperpolarized the fibres. High concentrations of magnesium ions or dramatic increase in the osmotic pressure of the bathing medium exerted no influence on the resting potential. It seems likely that skeletal muscle fibres of Acheta domesticus at rest are multionic electrodes, but the role played by various ions needs further studies.