Bacteriophage T7 genome has one transcription terminator sequence for the phageencoded RNA polymerase located after the gene 10 and named Tø. The factor-independent 41-bp terminator sequence along with its flanking sequences was cloned downstream of strong phage T7 promoters in various plasmids to construct efficient expression vectors. Its transcription termination efficiency was measured as the molar ratio of terminated transcripts to total transcripts produced from in vitro transcription reactions containing only purified T7 RNA polymerase, DNA template and ribonucleotides under standard conditions. The measured efficiency varied 17% to 78% depending on the sequences within transcriptional unit, but it does not depend on parental plasmid sequences. Futhermore, the efficiency appears to depend largely on the sequence immediately downstream of transcription initiation site, that is, early transcribed sequence. The genomic sequence immediately downstream of the initiation site can yield a 21-nucleotide RNA hairpin structure and causes the maximum termination efficiency. However, replacement with a similar hairpin-forming sequence at the same site reduced the efficiency substantially. Also removal of the genomic sequence does not change the efficiency. Thus, RNA hairpin structure itself is not responsible for the high efficiency.