ABSTRACT. – Ballast water in international navigation is deemed to play a key role in the unintentional movement of non-indigenous aquatic species across their geographical borderlines. In this context, the Gulf of Gabès is regarded as being exposed to species invasion due to its extensive human impact. The investigations carried out during the years 2009-2010 in the four port areas concerned 19 commercial ships. This study focused on the specific composition, status, abundance and viability of plankton in ballast water. For all ships, the qualitative analysis of samples was mainly marked by low diversity. The major groups were diatoms and Dinophyceae to a lower extent. The abundance of phytoplankton varied between 0 and 27.6 103 ind.dm–3. Most of the species found in ballast water were reported in different regions and were assumed to be cosmopolitan. The cell viability assessed by flow cytometry, revealed a viability rate ≤ 1 % for 85 % ships. Microfauna in the ballast water was very weakly represented and had a cosmopolitan distribution. No introduced species was detected. The quantity of sediment discharged in the Gulf of Gabès via ballast water during this period was described and estimated.