We have determined the kinetics of virus production and virus-specific RNA synthesis in Sac(−) cells infected with mouse hepatitis virus strain A59 (MHV-A59). Immunofluorescence showed that all cells became infected at a multiplicity of 10 PFU/cell. The virus was concentrated and purified to obtain the high titered stocks needed for these one-step growth experiments. Release of virus into the culture medium started 4 hr after infection (pi) and was complete at 10 hr pi. Synthesis of virus-specific RNA, measured by the incorporation of [3H]uridine in the presence of 1 μg/ml actinomycin D, also started at 4 hr pi and its maximum rate occurred between 6 and 8 hr pi. RNA labeled during this period was isolated from infected cells. About 50% of this RNA bound to oligo(dT)-cellulose; this material was denatured with glyoxal-dimethyl sulfoxide and analyzed by electrophoresis in 1% agarose gels. Seven RNA species with the following molecular weights were present: 5.6 × 106 (RNA1), 4.0 × 106 (RNA2), 3.0 × 106 (RNA3), 1.4 × 106 (RNA4), 1.2 × 106 (RNA5), 0.9 × 106 (RNA6), and 0.6 × 106 (RNA7). RNA1 comigrated with the viral genome. Artifacts caused by defective interfering particles or breakdown of RNA were excluded. To determine whether these RNA species were functional as messengers in infected cells, virus-specific RNAs present in polyribosomes were analyzed. EDTA treatment was used to discriminate between RNA present in polyribosomes and in EDTA-resistant, presumably ribonucleoprotein, particles. Most (91%) of RNA1 was present in EDTA-resistant particles; the remainder and all other RNAs synthesized between 6 and 8 hr pi were present in polyribosomes. We conclude that MHV-A59 has six subgenomic mRNAs. Since the total molecular mass (11.1 × 106 daltons) of these messengers is about twice that of the viral genome, sequence homologies must exist between the mRNAs. The position of these homologous regions and the translation products of each of the mRNAs remain to be determined.