In this paper, we report the construction in vitro of chimerae between lambdoid replacement vectors (Murray et al., 1977) and the miniF Apr plasmid: pSC138 (Timmis et al., 1975). lambdaF recombinants were shown to be chimerae between the lambda and the F replicons. By genetical tests, we have demonstrated that both lambda and F replication mechanisms are functional: the lambdaF recombinant behaves as a non defective plaque forming phage on lambda sensitive bacteria and establishes itself as a stable plasmid on recA F- homoimmune bacteria. In the extra-chromosomal state, the lambdaF recombinant apparently retains the controlled autonomous replication and the FI incompatibility characteristics of the F plasmid. The potential experimental uses of these phages are discussed.