The recombinant plasmid of pCU103 constructed by cloning pcbCD genes in pBluescript SK(+) was studied for the effect of temperature on its persistence in different waters by the methods of electrophoresis, Southern hybridization, quantification, and transformation. The plasmid was very rapidly degraded out in non-sterile FW water without regards to water temperature, probably due to the effect of biochemical factor such as nucleases. The pCU103 was most persistent at 4$^{circ}C$ in any water environments, moderately persistant at 15$^{circ}C$ but least stable at 3$0^{circ}C$ such results could be explained by the facts that hydrogen bonds in double-stranded plasmid DNAs become unstable and that nucleases are activated by increasing temperature. The intact structure of pCU1-3 was generally observed by gel electrophoresis under the conditions which the plasmid should be 2.0 ng/$mu extrm{l}$ or higher in concentration and that about 10$^2$ CFU/ml or more transformant cells should be recovered.