Two recombinants between the phage M13 and the plasmid pBR322 were isolated, analyzing the plasmid content of over one hundred colonies obtained by transduction. The study of the structure of both recombinants indicates that a fragment of the M13 genome has been integrated to pBR322. In both cases, the fragment contains a part of the phage replication region inserted either in the vicinity or within the pBR322 replicon. The fact that the phage and plasmid replicons seem to be involved in the recombination event suggests that it is helpful when the replication begins. So far it has not been possible to isolate a recombinant taking the whole genomes of pBR322 and M13. This is, undoubtedly, due to the instability of the recombinant molecule.