High-resolution single crystals of a catalytic RNA molecule derived from the sequence of the satellite RNA of tobacco ringspot virus have been obtained. The unit-cell volumes of the RNA crystals vary depending on the crystallization conditions and temperature. The best crystal form, when flash frozen, has space group P1 with unit-cell dimensions a = 53.08, b = 71.81, c = 28.03 A, alpha = 98.43, beta = 104.32 and gamma = 74.54 degrees. This form diffracts to a resolution of 2.4 A. A heavy-atom derivative search is in progress.