In vitro transcription of T3 DNA by T3 phage-induced RNA polymerase (nucleosidetriphosphate:RNA nucleotidyltransferase; EC 2.7.7.6) yields eight discrete RNAs (designated I-VIII) with molecular weights of approximately 6.2, 4.7, 4, 2.8, 1.8, 0.9, 0.52, and 0.21 X 10(6), respectively. Comparison of the size of in vitro T3 RNA polymerase transcripts with in vivo late T3 mRNAs indicates that several late RNAs produced in T3-infected cells do not correspond to any of the in vitro RNAs, and no RNAs as large as the three largest in vitro RNA species, I, II, and III, are observed. Escherichia coli RNase III cleaves these three high molecular weight T3 RNA polymerase transcripts to discrete RNAs that comigrate in polyacrylamide gel electrophoresis with some of the late T3 RNAs.