Galactokinase catalyses the conversion of galactose to galactose-1-phosphate as the first step in the Leloir pathway, a metabolic route that eventually enables the degradation of galactose via the glycolytic pathway. Galactokinases have been isolated from a wide range of prokaryotic and eukaryotic organisms and the enzyme has been identified as a member of the GHMP kinase (galactokinase, homoserine kinase, mevalonate kinase and phosphomevalonate kinase) superfamily. Pyrococcus furiosus galactokinase was cloned, expressed in Escherichia coli, purified and crystallized using the hanging-drop method of vapour diffusion with ammonium sulfate as the precipitant. The crystals belong to the space group C222(1), with more than eight subunits in the asymmetric unit and with approximate unit-cell parameters a = 211.7, b = 355.4, c = 165.5 A, alpha = beta = gamma = 90 degrees. The crystals diffract X-rays to 2.9 A resolution on a synchrotron-radiation source. Determination of the structure will provide insights into the molecular basis of substrate recognition and catalysis of this enzyme, for which no structures are currently available.