Phytoplanktonic diversity was determined in the coastal areas of Pacasmayo and Magdalena de Cao from May to October 2017. Six sampling stations were established in Pacasmayo and eight in Magdalena de Cao, with a monthly frequency. The phioplanktonic composition was determined by qualitative analysis and cell density with quantitative analysis. 93 species were recorded, represented by 58% of diatoms, with frequent species: Chaetoceros lorenzianus, Skeletonema costatum, Chaetoceros debilis, Thalassionema nitzschioides, Lithodesmiun undulatum, Coscinodiscus perforatus, Coscinodiscus wailessi, followed by 40% by dinoflagellates, represented mainly by the genera Protoperidinium and Ceratium, and 2% belonged to the silicoflagellates. The highest cell densities occurred in October with concentrations higher than 420 x104 cel / m3 associated with temperatures lower than 16.5 ° C. The highest av...