The stability of hybrid plasmids, constructed on the basis of vector pCV20(AprTcr) and containing HindIII fragments of Drosophila melanogaster DNA (pDm6, pDm9) and PstI fragments of D. melanogaster DNA (pDm39, pDm187, pDm189) was studied. After the transformation of E. coli HB101 recA and Escherichia coli 802 rec+ and selection to Tcr (pDm6, pDm9), or to Apr (pDm39, pDm189, pDm187) 0.04--9% of clones with reduced resistance to Tc or Ap was detected. The hybrid plasmids are more stable in rec-, but not in rec+ strain, the stability depends of the nature of cloned DNA, and on the site of vector DNA in which foreign genes are cloned. Restriction endonuclease analysis revealed that all plasmids of the clones with reduced Tcr or Apr lost the inserted DNA and the excision of foreign DNA occurred precisely in the sites of cloning. We suggest that the genome of the hybrid plasmid in the region of foreign insertion has a conformation which allows the bringing together the ends of cloned DNA with the following excision of the foreign genes.