MANY signalling cascades use seven-helical transmembrane receptors coupled to heterotrimeric G proteins (G(alpha beta gamma)) to convert extracellular signals into intracellular responses(1). Upon nucleotide exchange catalysed by activated receptors, heterotrimers dissociate into GTP-bound G(alpha) subunits and G(beta gamma) dimers, either of which can modulate many downstream effectors(2,3). Here we use multiwavelength anomalous diffraction data to solve the crystal structure of the beta gamma dimer of the G protein transducin. The beta-subunit is primarily a seven-bladed beta-propeller that is partially encircled by an extended gamma-subunit. The beta-propeller, which contains seven structurally similar WD repeats, defines the stereochemistry of the WD repeat and the probable architecture of all WD-repeat-containing domains. The structure details interactions between G protein beta- and gamma-subunits and highlights regions implicated in effector modulation for the conserved family of G protein beta gamma dimers.