We have constructed a cloning vector based on plasmid mini-F for use in Escherichia coli. Plasmid pZC320 consists of the ori-2 replication unit of F that confers very low copy number (lcn), and includes the sop partition functions to insure stable plasmid maintenance in the absence of selection. A multiple cloning site (MCS) containing 16 unique restriction sites is located within the 5' end of the lacZ alpha gene. Expression of lacZ alpha is under the control of the wild-type lactose operator/promoter (lacOP) region and is efficiently repressed by the lacI repressor. Clones containing inserts can be detected using the blue/white screen for beta-galactosidase (beta Gal). A T7 promoter allows transcription of cloned inserts in the presence of T7 RNA polymerase. We have demonstrated the use of this lcn vector for cloning the regulated tetracycline-resistance genes from Tn10, which confer only low-level resistance when present at high copy number.