In order to increase the molecular tools and markers needed for the identification of phytoplankton species, the inter simple sequence repeat ( ISSR) fingerprinting was adapted to micro-algae and its use in genetic analysis was demonstrated. Twelve strains, 6 Alexandrium, 4 Pseudo-nitzschia, 1 Skeletonema and 1 Tetraselmis were analysed for the first time with ISSR amplifications. The patterns were highly polymorphic and very reproducible. The 6 primers gave 223 polymorphic markers that clearly and easily distinguished all 12 strains ( mainly toxic ones) and gave 187 polymorphic markers among the Alexandrium and the Pseudo-nitzschia species. ISSR amplifications also indicated a large occurrence of simple sequence repeat ( SSR) in phytoplankton genomes, especially in Pseudo-nitzschia, and show their usefulness to cluster intra and inter species. ISSR markers were found to be good markers for genetic characterization and diversity study and led to consider them as new tools for the survey of phytoplankton.