The channel-forming colicins are plasmid-encoded bacteriocins that kill E. coli and related cells and whose mode of action is of interest in related problems of protein import and toxicology. Colicins parasitize metabolite receptors in the outer membrane and translocate across the periplasm with the aid of the Tol or Ton protein systems. X-ray structure data for the channel domain and colicin are available. Residues have been identified that affect the channel ion selectivity and particular helices implicated in channel structure and in conformational changes required for binding or insertion of the channel into the membrane. Unique aspects of the colicin channel system are the involvement of protein import in the gating process, the existence of multiple open and closed states, and the existence and action of an immunity protein that involves specific intramembrane helix-helix interactions with transmembrane helices of the colicin channel-forming domains.