Uroguanylin, which serves as an endogenous ligand of guanylyl cyclase C, is initially secreted in the form of a precursor, prouroguanylin. The N-terminal region of prouroguanylin interacts with the mature portion of prouroguanylin during the folding pathway. Here, a preliminary X-ray crystallographic study of prouroguanylin is presented. Prouroguanylin was refolded, purified and crystallized using the hanging-drop vapour-diffusion method. Prouroguanylin crystals were cryocooled and used for data collection. The diffraction data showed that the crystals belonged to space group P6(1)22, with unit-cell parameters a = b = 55.6, c = 157.7 A, and diffracted to 2.5 A resolution. The structure is currently being analyzed.