Transfer RNA (tRNA) molecules play a crucial role in protein biosynthesis in all organisms. Their interactions with ribosomes mediate the translation of genetic messages into polypeptides. Three tRNAs bound to the Escherichia coli 70S ribosome were visualized directly with cryoelectron microscopy and three-dimensional reconstruction, The detailed arrangement of A- and P-site tRNAs inferred from this study allows localization of the sites for anticodon interaction and peptide bond formation on the ribosome.