An IncP plasmid probe (pUCV2), coding for Cm-r, Km-r and bearing Ap (am) and Tc (am) resistance determinants was constructed by transposition of Tn9 (Cm-r) onto plasmid pLM2 for an efficient selection of potential recipients among natural marine bacteria. Using a Dap- E. coli donor, transmission of pUCV2 to marine bacteria was tested. pUCV2 is transferred to about 4-8% of natural, marine bacterial cells capable of forming colonies on a low nutrient, marine agar medium. The following bacterial genera, commonly found in the marine environment, could be detected when twenty of the transconjugant colonies obtained were identified: Vibrio, Pseudomonas and Aeromonas.