THE recent discovery of the ClC-family of anion-conducting channel proteins(1-3) has led to an appreciation of the central roles played by chloride ion channels in cellular functions, such as electrical behaviour of muscle(4-7) and nerve(8) and epithelial solute transport(9). Little is known, however, about molecular architecture or sequence-function relationships in these membrane proteins, In the single case of ClC-0, a voltage-gated ’muscle-type’ chloride channel, the functional complex is known to be a homo-oligomer of a polypeptide of M(r) similar to 90,000, with no associated ’helper’ subunits(10). The subunit stoichiometry of ClC-type channels is controversial, however, with either dimeric or tetrameric association suggested by different indirect experiments(10,11). Before a coherent molecular view of this new class of ion channels can emerge, the fundamental question of subunit composition must first be settled, We have examined hybrid ClC-0 channels constructed from functionally tagged subunits, and report here that ClC-0 is a homodimer containing two chloride-conduction pores.