The L-A virus (LAV) particle is a specialized compartment for the transcription and replication of double-stranded RNA. It is 390 Angstrom in diameter and infects yeast. The particle is formed by a capsid containing 120 copies of a 680-residue gene product arranged with T = I icosahedral symmetry, approximately two copies of an RNA-directed RNA polymerase, and a 4.6-kb linear, duplex RNA. LAV crystals diffracting to at least 4.5-Angstrom resolution were grown in a combination of polyethylene glycol 8000, ethylene glycol, and lithium chloride. Following crystallization the reservoir solution was replaced by a 2 X concentrated reservoir solution in order for ethylene glycol to function as a cryoprotectant even though initial crystals would not grow at sufficiently high concentrations of ethylene glycol for cryoprotection. A complete data set was collected to 6-Angstrom resolution from a frozen crystal obtained with this procedure. The crystals belong to space group P2(1). The unit cell dimensions are a = 406.7 Angstrom, b = 403.3 Angstrom, c = 572.5 Angstrom, beta = 90.3 degrees with two virus particles in the unit cell. The particle orientation was determined with the rotation function and the particle center was estimated on the basis of packing considerations.