Phytoplankton study was carried out at cockle culture area in Sg. Jarum Mas, Perak, from February to October 2008. A total of 64 water samples were collected from eight sampling stations. Using an inverted microscope, a total of eight genera of potentially harmful microalgae were identified belonging to two different classes; dinoflagellates and diatoms. The potentially harmful dinoflagellates observed were Alexandrium sp., Dinopyhsis sp., Dinophysis caudata, Prorocentrum sp., Ceratiumfurca, Noctiluca scintillans andAkashiwo sanguinea. Meanwhile the potentially harmful diatom observed was Pseudo-nitzchia sp. The enumeration of the cell density for each of the potential harmful microalgae have also been carried out. The results show that the density of cells for each of the potential harmful microalgae is low, but, Alexandrium sp. found relatively high concentrations in April of - 25, 000 cells / L. However, no cases of food poisoning due to marine microalgae or red tide incidents reported during the period of this study.