Transcription termination efficiencies of a phage T7 terminator $T{phi}$ for the T7 and SP6 RNA polymerases were studied. The terminator $T{phi}$ cloned in the plasmid pET-3 was subcloned into the plasmid pGEM3Z which contains both SP6 and T7 RNA polymerase promoters facing each other. Amounts of transcripts terminated at $T{phi}$ were compared with amounts of read-through transcripts run off at a restriction site. For quantitative analysis the transcripts were labeled with [${alpha}-^{32}P$]rNTP in in vitro transcription reaction. Transcription of T7 RNA polymerase terminates with efficiency of 78% by the $T{phi}$ in the pET-3, but terminates with efficiency of 38% by the $T{phi}$ in the pGEM3Z. It suggests that DNA sequences other than the terminator sequence affect the termination efficiency. Interestingly, the $T{phi}$ shows 81% termination efficiency for the SP6 RNA polymerase. Now this becomes the first strong terminator discovered for the SP6 RNA polymerase. This also suggests that both phage RNA polymerases might share similar mechanism of transcription termination.