The Shaker locus of Drosophila melanogaster encodes a family of A-type potassium channel subunits. Shaker mutants behave as antimorphs in gene dosage tests. This behaviour is due to the production of truncated A-channel subunits. We propose that they interfere with the function of their normal counterpart by forming multimeric A-channel structures. This hypothesis was tested by constructing transgenic flies carrying a heat-inducible gene encoding a truncated A-type potassium channel subunit together with a normal wild type doses of A-type potassium channel subunits. The altered subunit leads at larval, pupal or adult stages to the transformation of wild type into Shaker flies. The transformed flies exhibited a heat-inducible abnormal leg shaking behaviour and a heat-inducible facilitated neurotransmitter release at larval neuromuscular junctions. By the overexpression of an aberrant A-channel subunit the normal behaviour of transgenic D. melanogaster can be altered in a predictable way.