The copy number of the P1 plasmid replicon is stringently controlled, giving only one or two copies per newborn cell. Control is achieved by the action of the copy-control locus incA, which contains nine repeats of the 19-basepair binding site for the plasmid-encoded initiator protein RepA. A set of five similar repeats are present in the replication origin where RepA acts to trigger initiation. Using an in vitro replication system consisting of an Escherichia coli extract, the P1 origin as a template, and purified RepA protein, we show that supercoiled DNA circles containing the incA locus block origin function in trans. Shutdown becomes complete at a 1:1 ratio of origin to incA sequences. This is not due to titration of the RepA protein, as an excess of RepA can be added without restoring activity. Rather, the incA sequences appear to block the origin by direct contact in a plasmid-plasmid pairing event. When both the origin and the incA locus are present on one plasmid, trans contacts with daughter molecules appear to predominate over cis looping. The results are consistent with a model for replication control where daughter plasmids block their own replication by a pairing in which each origin is in contact with the incA locus of its partner.