Undecaprenyl diphosphate synthase from Micrococcus luteus B-P 26, one of the Z-prenyl chain-elongating enzymes, was crystallized using the sitting-drop vapour-diffusion method with ammonium sulfate and lithium sulfate as precipitants. The crystals belong to the monoclinic space group C2, with unit-cell parameters a = 127.2, b = 60.2, c = 75.7 A, beta = 105.6 degrees. The crystals diffract X-rays to at least 2.2 A resolution using synchrotron radiation and are suitable for high-resolution crystal structure analysis.