Using the whole‐cell recording patch clamp tehnique in clonal cultures of human muscle satellite cells (SC), we studied a voltage‐gated potassium current analogous to the delayed rectifier current (I _kdr ) described in adult human skeletal muscle. This current was absent in proliferating SC cultured in a growth medium containing 15% serum, except when the SC approached the end of their replicative life (between 77 and 124 days in culture); at that time, approximately 50% of the SC possessed I _kdr In contrast, I _kdr was expressed within less than 4 days in approximately 70% of the SC cultured in a serum‐free medium (SFM) and within 24 h in differetiating medium. We belive that I _kdr may be a characteristic feature of fusion‐component SC and that it may be involved in the fusion process for the following resons: (1) after the transfer in differentiating medium, cultures of SC in which the expression of I _kdr was previously promoted by exposure to SFM were found to fuse immediately, without the initial 24 h lag time observed in control sister cultures; (2) in the latter “naive” SC, I _kdr was expressed during the first day in differentiating medium, before SC began to fuse; (3) most of the SC that did not fuse even after weeks of exposure to differentiating medium did not express I _kdr ; (4) TEA, at a concentration of 3 mM, reduces the amplitude of I _kdr by 55% and the fusion index by 55–67%. © 1995 Wiley‐Liss, Inc.