In the in vitro transcription reactions limiting concentrations of a ribonucleotide cause the phage SP6 RNA polymerase to pause long enough specifically only at the positions of the limited nucleotide and dissociate from the elongation complex. As a result a series of RNA oligomers comprising a sequencing ladder was obtained in high-resolution polyacrylamide-urea gel electrophoresis. Also, RNA oligomers up to 6-mer were made not only nucleotide-specifically, but also non-specifically. According to these results, the phage SP6 RNA polymerase appears to escape from the abortive initiation cycling after synthesizing 6 base long RNA, then get into the stable elongation mode as 7 base long RNA is made.