Previous research in Drosophila has implied that a variety of mutations affect ion channels1-4. In one of these studies it was suggested that mutations mapping at the X-linked Shaker locus of D. melanogaster might cause defects in some type of potassium channel in larval motoneurones2. We show here, by voltage-clamp analysis, that mutations at the Shaker locus affect one of the two types of voltage-sensitive potassium channels present in the Drosophila dorsal longitudinal flight muscle (DLM). Initial voltage-clamp studies5,6 revealed that one set of channels carries a fast, transient outward current, similar to the molluscan A current7-9, that is altered by mutations at the Shaker locus. The other set of channels carries a slower activating current, similar to delayed rectification 10,11, that is not affected by the Shaker mutation.