We have analyzed the effect of flanking sequences on Tus-induced replication arrest. pBR322 plasmid derivatives which carry the Escherichia coli replication terminator TerB at different locations were used. Efficiency of the replication arrest was estimated from the plasmid copy number and transformation frequency of tus+ cells. We found that flanking sequences do affect replication arrest efficiency, a weak arrest being correlated with the presence of an AT-rich region which is replicated just before TerB. Some sequences located after the replication terminator can also affect replication termination. We propose that the AT-rich regions might impair binding of the Tus protein to the TerB sequence or facilitate helicase-induced unwinding of DNA and Tus displacement from the TerB site.