Phosphoinositides now emerge as important regulators of membrane traffic. In particular, phosphatidylinositol 4-phosphate may serve as a precursor for polyphosphorylated derivatives of phosphatidylinositol and, also, may regulate vesicular traffic by recruiting specific proteins to the membrane. Early results have demonstrated the presence of phosphatidylinositol 4-kinase (PI4K) activity in glucose transporter 4 (Glut4) vesicles from fat and skeletal muscle cells. However, the molecular identity of phosphatidylinositol 4-kinase(s) associated with Glut4 vesicles has not been characterized. It has also been determined that Glut4 vesicles are not homogeneous and represent a mixture of at least two vesicular populations: ubiquitous cellugyrin-positive transport vesicles and specialized cellugyrin-negative insulin-responsive Glut4 storage vesicles, which are different in size, protein composition, and functional properties. Using sequential immunoadsorption, subcellular fractionation, and immunofluorescence staining, we show that virtually all PI4K activity in Glut4 vesicles is represented by PI4K type IIalpha, which is associated with cellugyrin-positive vesicles and is not detectable in the Glut4 storage vesicles. The unique N terminus of PI4K type IIalpha is required for the targeting of the enzyme to cellugyrin-positive vesicles. Knockdown of PI4K type IIalpha with the help of short hairpin RNA does not decrease the amount of cellugyrin-positive vesicles in human embryonic kidney 293 cells.