The eag family of K+ channels contains three known subtypes: eag, elk, and erg. Genes representing the first two subtypes have been identified in flies and mammals, whereas the third subtype has been defined only by the human HERG gene, which encodes an inwardly rectifying channel that is mutated in some cardiac arrhythmias. To establish the predicted existence of a Drosophila gene in the erg subfamily and to learn more about the structure and biological function of channels within this subfamily, we undertook a search for the Drosophila counterpart of HERG. Here we report the isolation and characterization of the Drosophila erg gene. We show that it corresponds with the previously identified seizure (sei) locus, mutations of which cause a temperature-sensitive paralytic phenotype associated with hyperactivity in the flight motor pathway. These results yield new insights into the structure and evolution of the eag family of channels, provide a molecular explanation for the sei mutant phenotype, and demonstrate the important physiological roles of erg-type channels from invertebrates to mammals.