Cotranslational translocation of proteins requires ribosome binding to the Sec61p channel at the endoplasmic reticulum (ER) membrane. We have used electron cryomicroscopy to determine the structures of ribosome-channel complexes in the absence or presence of translocating polypeptide chains. Surprisingly, the structures are similar and contain 3-4 connections between the ribosome and channel that leave a lateral opening into the cytosol. Therefore, the ribosome-channel junction may allow the direct transfer of polypeptides into the channel and provide a path for the egress of some nascent chains into the cytosol. Moreover, complexes solubilized from mammalian ER membranes contain an additional membrane protein that has a large, lumenal protrusion and is intercalated into the wall of the Sec61p channel. Thus, the native channel contains a component that is not essential for translocation.