Phytoplankton is a primary producer organism in aquatic ecosystem and it supplies oxygen to the waters through photosynthesis process. A research aim to determine types and abundance of phytoplankton in the PTPN V Sei Pagar Reservoir has been carried out on July-August 2019. Sampling taken at three stations, namely station 1 (inlet area), station 2 (central area) and station 3 (outlet area…