The heterotrimeric Sec61p complex is a major component of the protein-conducting channel of the endoplasmic reticulum (ER) membrane, associating with either ribosomes or the Sec62/63 complex to perform co- and posttranslational transport, respectively. We show by electron microscopy that purified mammalian and yeast Sec61p complexes in detergent form cylindrical oligomers with a diameter of ∼85 Å and a central pore of ∼20 Å. Each oligomer contains 3–4 heterotrimers. Similar ring structures are seen in reconstituted proteoliposomes and native membranes. Oligomer formation by the reconstituted Sec61p complex is stimulated by its association with ribosomes or the Sec62/63p complex. We propose that these cylindrical oligomers represent protein-conducting channels of the ER, formed by ligands specific for co- and posttranslational transport.