The ribosome is formed by assembly of proteins and nucleic acids, and synthesizes proteins according to genetic instructions in all organisms. Many of the biochemical steps of this fundamental process are known, but a detailed understanding requires a well-defined structural model of the ribosome. Electron microscopy combined with image reconstruction of two-dimensional crystals or single ribosomes has been the most promising technique, but the resolution of the resulting models has been insufficient. Here we report a 25-A reconstruction of the ribosome from Escherichia coli, obtained by combining 4,300 projections of ice-embedded single particles. Our new reconstruction reveals a channel in the small ribosomal subunit and a bifurcating tunnel in the large subunit which may constitute pathways for the incoming message and the nascent polypeptide chain, respectively. Based on these new findings, a three-dimensional model of the basic framework of protein synthesis is presented.