1. The temperature-dependent component of the resting potential in intact, cannulated and dialysed fibres from the muscle of the barnacle Balanus nubilus was studied under a variety of different experimental conditions. A decrease in temperature from 22 to 12 degrees C produced a mean depolarization of 10 mV.2. Neither addition of strophanthidin, nor replacement of external sodium by lithium affect the voltage shift induced by temperature. However, the magnitude of the voltage shift depends on the external chloride and potassium concentration.3. The dialysis technique was applied to measure the potassium, chloride and sodium fluxes as a function of temperature. The Q(10) for the passive fluxes of these ions was 1.9, 1.7, and 1.4 respectively.4. The temperature-dependent changes in the passive ionic fluxes combined with the inability of inhibitors of the sodium pump to alter the temperature dependence of the resting potential suggest that the change induced by temperature on the resting potential is primarily caused by a change in the passive permeability ratios, and is not related to active ion transport.