Crystals of extracellular fragment X3 of a human sperm membrane protein YWK-II/APPH have been grown at 291 K using 8% PEG 4000 as precipitant by the vapour-diffusion method. The diffraction pattern of the crystal extends to 2.9 A resolution at 100 K using Cu Kalpha radiation in-house. The crystals belong to space group P2(1), with unit-cell parameters a = 46.0, b = 43.7, c = 90.2 A, alpha = gamma = 90.0, beta = 106.6 degrees. Furthermore, a selenomethionine (SeMet) derivative of the protein was overexpressed in the same expression system and was purified in a reducing environment. The derivative crystals were obtained under similar conditions. Subsequently, a single-wavelength data set was collected to 2.38 A resolution from the derivative crystal at ESRF. The crystals belong to space group P2(1), with unit-cell parameters a = 46.2, b = 44.0, c = 88.3 A, alpha = gamma = 90.0, beta = 103.6 degrees. The presence of one molecule per asymmetric unit gives a crystal Volume per protein mass (V(M)) of 2.8 A(3) Da(-1) and a solvent content of 56.4% by Volume.