THE RNA bacteriophage MS2 is a convenient model system for the study-of protein-RNA interactions, The MS2 coat protein achieves control of two distinct processes-sequence-specific RNA encapsidation and repression of replicase translation-by binding to an RNA stem-loop structure of 19 nucleotides containing the initiation codon of the replicase gene. The binding of a coat protein diner to this hairpin shuts off synthesis of the viral replicase(1), switching the viral replication cycle to virion assembly rather than continued replication. The operator fragment alone can trigger self-assembly of the phage capsid at low protein concentrations and a complex of about 90 RNA operator fragments per protein capsid has been described(2). We report here the crystal structure at 3.0 Angstrom resolution of a complex between recombinant MS2 capsids and the 19-nucleotide RNA fragment. It is the first example of a structure at this resolution for a sequence-specific protein-RNA complex apart from the transfer RNA synthetase complexes(3-5). The structure shows sequence-specific interactions between conserved residues on the protein and RNA bases essential for binding.