The delayed rectifier K+ current in Drosophila is similar to the classical delayed rectifier, originally described by Hodgkin and Huxley. Drosophila provides unique tools of mutant analysis to unambiguously determine the genetic identity of this native K+ current. We identified the Shab gene as the exclusive gene underlying delayed rectifier currents in both muscle and neurons. In muscles, a genetic mutation of Shab removes virtually all the whole cell delayed rectifier current (IK), while leaving unaltered the transient A-current encoded by the Shaker gene. In neurons, the Shab mutation also removes the bulk of IK, but leaves unaltered the transient A-current encoded by the Shal gene. Although most of the delayed rectifier current is the product of the Shab gene, the Shaw gene contributes a small “leak” current to most neurons and muscle cells. Thus, in contrast to the A-currents which are encoded by different genes in muscle and neuronal cell bodies (Shaker and Shal, respectively), the predominant IK in both muscle and neurons is encoded by the same gene (Shab). With the genetic identity of IK confirmed, all of the major K+ currents in embryonic Drosophila neurons and muscle are now known.