Escherichia coli contains two phosphofructokinases, Pfk-1 and Pfk-2, which belong to unrelated protein families. In addition to catalytic function, the enzymes have converged in showing substrate inhibition by the nucleotide MgATP. However, although both Pfk-1 and Pfk-2 have been extensively characterized biochemically, only the structure of the former has been solved by X-ray diffraction. In order to fully understand how the same function has evolved on different structural folds, Pfk-2 has been crystallized by the hanging-drop vapour-diffusion method using PEG 6000 as precipitant. Single crystals were grown in the presence of MgATP and diffracted to 1.98 A. The crystals belong to the orthorhombic system, space group P222(1), with unit-cell parameters a = 42.8, b = 86.8, c = 171.3 A. The calculated Matthews coefficient of 2.45 A(3) Da(-1) indicates the presence of two monomers in the asymmetric unit, corresponding to a solvent content of 49%. Structure determination is ongoing.