Old yellow enzyme (OYE) is an NADPH oxidoreductase that contains a flavin mononucleotide as a prosthetic group. The OYE from Trypanosoma cruzi, which produces prostaglandin F(2alpha), a potent mediator of various physiological and pathological processes, from prostaglandin H2. The protein was recombinantly expressed and purified from Escherichia coli and was crystallized using the hanging-drop vapour-diffusion method. The crystal belongs to the monoclinic space group P2(1), with unit-cell parameters a = 56.3, b = 78.8, c = 78.8 A, beta = 93.4 degrees and two molecules per asymmetric unit. The crystals were suitable for X-ray crystallographic studies and diffracted to 1.70 A resolution. A Patterson search method is in progress using the structure of OYE from Pseudomonas putida as a starting model.