The seasonal cycle of phytoplankton with emphasis on Pseudo-nitzschia was studied at Elle, Drøbak from September 2016 to August 2017. Vertical (1 m) and horizontal net hauls (20 – 0 m), and water samples (1 m) were collected monthly for qualitative and quantitative analysis. The net hauls were preserved with Lugol’s solution for further species identification under light microscopy (LM). Additional net haul samples were preserved with formalin for species identification under scanning (SEM) and transmission electron microscopy (TEM). The water samples collected from surface water were preserved with Lugol’solution for further quantitative examinations using the Uthermöhl method and an inverted microscope. During the sampling year, 73 species have been identified. The samples were dominated by diatoms (40 species) and dinoflagellates (30 species). The quantitative analysis revealed a high concentration of phytoplankton observed during spring. The phytoplankton abundance was very low during the winter period with 7160 Cells/L counted while during the spring, a maximum concentration of 8 593 000 Cells/L was recorded in May. The phytoplankton abundance was mainly dominated by Pseudo-nitzschia species that accounted for 96.91% of the total phytoplankton and 97% of the total diatoms. From the TEM and SEM analysis, eight Pseudo-nitzschia species were identified: P. fraudulenta (Cleve), P. pseudodelicatissima (Hasle), P. cuspidata (Hasle), P. multiseries (Hasle), P. calliantha (Lundholm, Moestrup et Hasle), P. pungens (Grunow ex P.T. Cleve), P. delicatissima (Lundholm, Moestrup et Hasle) and P. seriata (Cleve). The species of Pseudo-nitzschia identified were all categorized to be potentially toxic and able to produce the toxin domoic acid. Species identification was based on the morphological measurement performed by Hasle in 1996.