Billions of tons of ballast water are lreased world-wide every year. Living marine, brackish or freshwater organisms, including those noxioux or unwanted, can travel by that way from a part of the world to another. A successful introduction needs : 1) survival during the pumping process 2) survival in ships' ballasts 3) survival and repreoduction in the new environment. To assess the survival conditions in ship's ballasts, a small scale pilot system has been built (MARTOB project), more realistic than laboratory vials and easier to handle than a real ballast. Bacteria and phytoflagellates coming from ship's sampling or from cultures were introduced in the system ; their survival in several waters or sediments are studied, a useful method to assess the efficiency of further treatment processes at small scale and thus low cost.