High-resolution structures of S. aureus D-tagatose-6-phosphate kinase (LacC) in two crystal forms are reported. The structures define LacC in apo form, in binary complexes with ADP or the non-hydrolysable co-factor analogue AMP-PNP and in a ternary complex with AMP-PNP and D-tagatose-6-phosphate. The tertiary structure of the LacC monomer, which is closely related to other members of the pfkB subfamily of carbohydrate kinases, is composed of a large / core domain and a smaller, largely 'lid'. Four extended polypeptide segments connect these two domains. Dimerization of LacC occurs via interactions between two lid domains, which come together to form a -clasp structure. Residues from both subunits in the dimer are required for substrate binding. LacC adopts a closed structure required for phosphoryl transfer only when both substrate and cofactor are bound. A reaction mechanism similar to that used by other phosphoryl transferases is proposed although, unusually, when both substrate and co-factor are bound to the enzyme two Mg²⁺ ions are observed in the active site. A new motif of amino acid sequence conservation common to the pfkB subfamily of carbohydrate kinases is identified.