Diacylglycerol kinase catalyses the ATP-dependent phosphorylation of diacylglycerol to phosphatidic acid for use in shuttling water-soluble components to membrane-derived oligosaccharide and lipopolysaccharide in the cell envelope of Gram-negative bacteria(1). For half a century, this 121-residue kinase has served as a model for investigating membrane protein enzymology(1-6), folding(7,8), assembly(9-12) and stability(1,13). Here we present crystal structures for three functional forms of this unique and paradigmatic kinase, one of which is wild type. These reveal a homo-trimeric enzyme with three transmembrane helices and an amino-terminal amphiphilic helix per monomer. Bound lipid substrate and docked ATP identify the putative active site that is of the composite, shared site type. The crystal structures rationalize extensive biochemical and biophysical data on the enzyme. They are, however, at variance with a published solution NMR model(14) in that domain swapping, a key feature of the solution form, is not observed in the crystal structures.