We established a quantitative hybridization system by which three types of influenza yin, RNAs (vRNA, mRNA, and cRNA) for the 8 genome segments were measured individuall As the hybridization probes, ³²P-labeled RNAs of both plus and minus polarity wet produced employing an SP-6 transcription system and used in a large molar owes; sufficient to overcome complementary RNAs present in the viral RNA samples. Employin the system, we studied the control of the synthesis of each viral RNA species in MDCK cell infected with A/Udorn/72 (H3N2). Our new observations were as follows. 1) Segment specific transcription was observed at the primary transcription. 2) Replication of the viru genome began simultaneously for all segments. No delay was observed in the replication c the segments carrying late genes. 3) In addition to control at the transcriptional levels, th expression of viral late genes was regulated at some post-transcriptional step(s). Thes results are not compatible with the concepts reported previously, and lead us to propos unique regulations operating on the expression of the viral late genes.