Elongation factor G (EF-G) is a GTPase that is involved in the translocation of bacterial ribosomes along messenger RNA during protein biosynthesis. In contrast to current models, EF-G-dependent GTP hydrolysis is shown to precede, and greatly accelerate, the rearrangement of the ribosome that leads to translocation. Domain IV of the EF-G structure is crucial for both rapid translocation and subsequent release of the factor from the ribosome. By coupling the free energy of GTP hydrolysis to translocation, EF-G serves as a motor protein to drive the directional movement of transfer and messenger RNAs on the ribosome.