High specificity of Tn9 transposition into the cryptic plasmid pCA25 of Erwinia carotovora subsp. carotovora 48A was established on the basis of the comparative restriction analysis of DNA by endonucleases EcoRV and PstI. Under the extremal conditions of growth (1% SDS) the plasmid pCA25:: Tn9 loses about 50 % of transpozone sequence, however the cells are not healed of extrachromosomal DNA. The inheritance stability, side by side with site-specific transposition and deleting of Tn9, confirms a supposition on the prophage origin of extrachromosomal element pCA25 of E. carotovora subsp. carotovora 48A.