Crystals of a diatom-infecting virus (CtenRNAV) that diffracted to a resolution of 4.0 A were grown in a mixture of 2-methyl-2,4-pentanediol (MPD), calcium chloride and sodium acetate. It was possible to freeze the crystals directly at liquid-nitrogen temperature as the reservoir solution, which included about 30% MPD, acted as a cryoprotectant during X-ray diffraction data collection. A data set was collected from a single frozen crystal obtained using this method. The crystals belonged to space group P6(3)22, with unit-cell parameters a = b = 448.67, c = 309.76 A and two virus particles in the unit cell. The virus-particle orientation was determined using a rotation function and the virus-particle centre was estimated on the basis of crystallographic considerations. The packing of CtenRNAV in the crystal lattice was revealed by this preliminary crystallographic study.