In the membrane of the flight muscle cells of developing Drosophila a large calcium-sensitive potassium current, IKc, was found. It was present before the development of voltage-activated potassium channels and seems to be the first potassium current to develop in the membrane. Also present in these early cells were large numbers of occult (hidden) calcium channels, which remained inactive until the end of pupal development. These inactive calcium channels could be made to function by injecting adenosine triphosphate or ethyleneglycol tetraacetic acid into the early cells. IKc has kinetic properties resembling the later developing voltage-sensitive current IKv, and is distinct from the fast, transient calcium-dependent outward current IAC, which appears much later in development. IAc closely resembles the voltage-sensitive current IAv, also present in these cells. Thus, both of the voltage-sensitive potassium channel types, IAv and IKc, have similar calcium-sensitive counterparts, IAc and IKc, that are present in the same cells.