ALUMINIUM fluoride (A1F⁻₄) activates members of the hetero-trimeric G-protein (G_αβγ) family^1,2 by binding to inactive G_α·GDP near the site occupied by the γ-phosphate in G^α·GTP (ref. 3). Here we describe the crystal structure of transducin a o GDP acti-vated with aluminium fluoride (G_tα·GDP·A1F⁻₄·H·O) at 1.7 Å, a resolution sufficient to establish the coordination geometry of the bound aluminium fluoride as well as the extensive network of direct and water-mediated interactions that stabilize it. These observations are derived from three independent representations in the asymmetric unit, eliminating any chance of drawing conclu-sions based on stereochemistry imposed by crystal packing. Surprisingly, aluminium fluoride activates G_α·GDP by binding with a geometry resembling a pentavalent intermediate for GTP hydrolysis. The stabilizing interactions involve not only residues that interact with the y-phosphate in G_tα·GTPγS, but also conserved residues essential for GTPase activity. Thus the G_tα·GDP·AIF⁻₄·H₂O structure provides new insight into the mechanism of GTP hydrolysis.