1. Using the patch-clamp technique, a new non-inactivating voltage-gated potassium current, IK(ni), was studied in cultured fusion-competent human myoblasts. 2. IK(ni) is activated at voltages above -50 mV and its conductance reaches its maximum around +50 mV. Once activated, the current remains at a steady level for minutes. 3. Reversal potential measurements at various extracellular potassium concentrations indicate that potassium ions are the major charge carriers of IK(ni). 4. IK(ni) is insensitive to potassium channel blockers such as charybdotoxin, dendrotoxins, mast cell degranulating (MCD) peptide, 4-aminopyridine (4-AP), 3,4-diaminopyridine (3,4-DAP) and apamin, but can be blocked by high concentrations of TEA and by Ba2+. 5. A potassium channel of small conductance (8.4 pS at +40 mV) with potential dependence and pharmacological properties corresponding to those of IK(ni) in whole-cell recording is described. 6. IK(ni) participates in the control of the resting potential of fusion-competent myoblasts, suggesting that it may play a key role in the process of myoblast fusion.