Elongation factor G (EF-G) is a G protein factor that catalyzes the translocation step in protein synthesis on the ribosome. Its GTP conformation in the absence of the ribosome is currently unknown. We present the structure of a mutant EF-G (T84A) in complex with the non-hydrolysable GTP analogue GDPNP. The crystal structure provides a first insight into conformational changes induced in EF-G by GTP. Comparison of this structure with that of EF-G in complex with GDP suggests that the GTP and GDP conformations in solution are very similar and that the major contribution to the active GTPase conformation, which is quite different, therefore comes from its interaction with the ribosome.