The scope of this study was to explore the diversification of the phytoplankton community structure and related ecological determinants along a large scale (3188 km) longitudinal transect, covering the region from eastern Mediterranean to Western Basin. Sampling was performed during the period June – July of 1999. Six stations were located in the Eastern Basin, one station in the Sicily strait and two stations in the Western Basin.A total of 216 species were identified(48 diatoms, 112 dinoflagellates and 56 coccolithophores).