Single-channel currents of an anion-selective channel in the plasma membrane of cultured rat muscle cells (myotubes) were recorded with the patch-clamp technique (Hamill, O.P., A. Marty, E. Neher, B. Sakmann, and F.J. Sigworth, 1981. Pfluegers Arch. Eur. J. Physiol., 391:85-100). The channel is selective for Cl- over cations, and has an unusually large single-channel conductance of approximately 430 pS in symmetrical 143 mM KCl. The channel is often active at 0 mV, opening and closing spontaneously. When active, steps from 0 mV to either negative or positive membrane potentials close the channel to an apparent inactivated state. The mean effective time that a channel is open before it inactivates is approximately 1.19 s for steps to -30 mV and 0.48 s for steps to +30 mV. Returning the membrane potential to 0 mV results in recovery from inactivation. Calcium ions are not required for channel activity.