Three different types of impairment in the synthesis of virion RNA (vRNA) were detected in three groups of temperature-sensitive (ts) mutants of fowl plague virus (FPV) having ts mutations in genes 1, 3 and 5 respectively. Normal synthesis of poly-(A+) cRNA, poly(A-) cRNA and vRNA was observed under non-permissive conditions early in infection in cells infected with the ts43 mutant having a ts mutation in gene 1 coding for the PB2 protein. However, 4 h after infection synthesis of vRNA ceased, synthesis of poly(A+) cRNA was reduced drastically, but the rate of poly(A-) cRNA synthesis was the same as that in cells infected with wild-type FPV. In cells infected with the ts 166 mutant having a ts mutation in gene 3, coding for the PA polypeptide, a drastic reduction was observed in poly(A+) cRNA synthesis under non-permissive conditions. Synthesis of poly(A-) cRNA was also reduced and synthesis of vRNA was not detected. The ts 60 mutant, having a ts mutation in gene 5 coding for the NP polypeptide, induced synthesis of all types of virus-specific RNA under non-permissive conditions, but the regulation of synthesis of vRNA and poly(A+) cRNA was affected, there being predominant syntheses of RNA segments 5 and 8 late in infection. In cells infected with mutants ts43 and ts 166 synthesis of virus-specific proteins was impaired, which reflected defects in the synthesis of virus-specific RNAs. The data obtained suggest that the PB2 protein may be contained in an enzyme complex responsible for synthesis of vRNA, that different enzyme complexes may be involved in the synthesis of poly(A-) cRNA and vRNA, and that the NP protein plays a significant role in the regulation of vRNA synthesis.