The proteins Cdc42 and Rac are members of the Rho family of small GTPases (G proteins), which control signal-transduction pathways that lead to rearrangements of the cell cytoskeleton, cell differentiation and cell proliferation. They do so by binding to downstream effector proteins(1). Some of these, known as CRIB (for Cdc42/Rac interactive-binding) proteins(2), bind to both Cdc42 and : Rac, such as the PAK1-3 serine/threonine kinases(3), whereas others are specific for Cdc42, such as the ACK tyrosine kinases(4,5) and the Wiscott-Aldrich-syndrome proteins (WASPs)(6,7). The effector loop of Cdc42 and Rac (comprising residues 30-40, also called switch I), is one of two regions which change conformation on exchange of GDP for GTP, This region is almost identical in Cdc42 and Racs, indicating that it does not determine the specificity of these G proteins. Here we report the solution structure of the complex of Cdc42 with the GTPase-binding domain of ACK(4,5). Both proteins undergo significant conformational changes on binding, to form a new type of G-protein/effector complex. The interaction extends the beta-sheet in Cdc42 by binding an extended strand from ACK, as seen in Ras/effector interactions(8,9), but it also involves other regions of the G protein that are important for determining the specificity of effector binding.