Octaprenyl pyrophosphate synthase (OPPs) catalyzes the condensation of five isopentenyl pyrophosphates with farnesyl pyrophosphate to generate C(40) octaprenyl pyrophosphate. The enzymes from the hyperthermophilic bacterium Thermotoga maritima and from the mesophilic Escherichia coli were expressed in E. coli and the recombinant proteins were purified and crystallized. The T. maritima OPPs crystals belong to space group P42(1)2, with unit-cell parameters a = b = 151.53, c = 69.72 A. The E. coli OPPs crystals belong to space group C222(1), with unit-cell parameters a = 247.66, b = 266.10, c = 157.93 A. Diffraction data were collected at 100 K using synchrotron radiation and an in-house X-ray source. Structure determination of T. maritima OPPs has been carried out using MIR data sets at 2.8 A resolution. The asymmetric unit contains one dimer. An initial model with 280 residues per subunit has been built and refined to 2.28 A resolution. It shows mostly helical structure and resembles that of avian farnesyl pyrophosphate synthase.