Genomic recoding by A-->I RNA editing plays an important role in diversifying the proteins involved in electrical excitability. Here, we describe editing of an intronless potassium channel gene. A small region of human K(V)1.1 mRNA sequence directs efficient modification of one adenosine by human adenosine deaminase acting on RNA 2 (hADAR2). Mutational analysis shows that this region adopts a hairpin structure. Electrophysiological characterization reveals that the editing event (I/V) profoundly affects channel inactivation conferred by accessory beta subunits. Drosophila melanogaster Shaker channels, mimicking this editing event through mutation, exhibit a similar effect. In addition, we demonstrate that mRNAs for the paralogous D. melanogaster Shab potassium channel are edited at the same position by fly ADAR-a clear example of convergent evolution driven by adenosine deamination. These results suggest an ancient and key regulatory role for this residue in K(V) channels.