RNA degradation is important in the post-transcriptional control of gene expression. The processing, degradation and quality control of RNA is performed by many different classes of ribonucleases. Ribonuclease II (RNase II) is a 643-amino-acid enzyme that degrades single-stranded RNA from its 3'-end, releasing ribonucleoside 5'-monophosphates. RNase II was expressed both as the wild type and as a D209N mutant form. The latter was also produced as an SeMet derivative. The various protein forms were crystallized using the vapour-diffusion method. Wild-type RNase II was crystallized in two crystal forms, both of which belonged to space group P2(1). X-ray diffraction data were collected to 2.44 and 2.75 angstroms resolution, with unit-cell parameters a = 56.8, b = 125.7, c = 66.2 angstroms, beta = 111.9 degrees and a = 119.6, b = 57.2, c = 121.2 angstroms, beta = 99.7 degrees, respectively. The RNase II D209N mutant gave crystals that belonged to space group P6(5), with unit-cell parameters a = b = 86.3, c = 279.2 angstroms, and diffracted to 2.74 angstroms. Diffraction data from the mutant and its SeMet derivative enabled the determination of a partial Se-atom substructure by SIRAS.