The yeast phosphatidytinositol-transfer protein (Sec14) catalyses exchange of phosphatidytinositol and phosphatidylcholine between membrane bilayers in vitro. In vivo, Sec14 activity is essential for vesicle budding from the Golgi complex. Here we report a three-dimensional structure for Sec14 at 2.5 Å resolution. Sec14 consists of twelve α-helices, six β- strands, eight 310-helices and has two distinct domains. The carboxy- terminal domain forms a hydrophobic pocket which, in the crystal structure, is occupied by two molecules of n-octyt-β-D-glucopyranoside and represents the phospholipid-binding domain. This pocket is reinforced by a string motif whose disruption in a sec14 temperature-sensitive mutant results in destabilization of the phospholipid-binding domain. Finally, we have identified an unusual surface helix that may play a critical role in driving Sec14-mediated phospholipid exchange. From this structure, we derive the first molecular dues into how a phosphatidylinositol-transfer protein functions.