Quantitative and qualitative distribution of surface phytoplankton, as related to hydrographic conditions, was studied in the Gulf of Naples in February 1979. Previous work has shown that the Gulf of Naples is a diversified ecosystem, due to geographic and hydrographic features as well as man made eutrophication, that can be subdivided into two major parts: a coastal subsystem and an open water one. Hydrographic analysis of the winter situation at the surface fully confirms this picture, as it identifies two distinct water masses corresponding respectively to surfaced Tyrrhenian Intermediate Water and to Coastal Surface Water. The structural analysis of phytoplankton reveals three assemblages of species characterizing different water types: 1 the Ischia and Procida channels affected by the advection of Volturno river and Cuma outfall plumes; 2 the coastal area of the Gulf proper, namely the bays of Pozzuoli, Naples and Castellammare; 3 the open waters located beyond the 100 m isobath. The channel area assemblage is dominated by diatoms, particularly by fast growing species, such as Asterionella japonica, several species of Chaetoceros and Rhizosolenia hebetata f. semispina. The coastal assemblage is identified, among others, by the diatoms Cerataulina bergonii, Hemiaulus sinensis; the dinoflagellates Glenodinium lenticula, Exuviaella compressa and Porella perforata. The open water assemblage is characterized by the diatoms Coscinodiscus curvatulus and Hemidiscus cuneiformis, the dinoflagellate Amphidinium acutissimum and the coccolithophore Coccolithus haeckelii.