The gating of potassium ion channels has been shown to be dependent on voltage1 or Ca2+ ions2,3 or both4,5. A fast transient potassium current (sometimes denoted IA) is found in a wide variety of animals6-14. The Ca2+-sensitivity of this early outward current has been a matter of dispute as reports from different systems have indicated complete insensitivity6-8, marked sensitivity10-12 or only partial sensitivity13,14. It is possible that there are two distinct early outward current systems, one Ca 2+-sensitive and the other not. Thus, the reports of partial Ca 2+-sensitivity would indicate the presence of both systems in the membrane. I now report that in the adult Drosophila flight muscles, the transient outward current is also partially Ca2+-sensitive. The hypothesis that two separate currents are present is strengthened by the discovery that in several mutants of the X-linked Shaker locus (Sh KS133, Sh102 and ShK0120; refs15-17) the Ca2+-independent component (IAci) is absent with only the Ca2+-dependent component (IAcd) remaining. Hence, the mutations seem to delete one of two separate current systems. An alternative hypothesis is that only one channel type is present that can be modified by mutation to be totally Ca2+-dependent.