Phytoplankton are primary producers in aquatic ecosystems, their abundance is determined by environmental changes, so phytoplankton are often used as bioindicators of waters. The aim of this research is to determine the diversity and abundance of phytoplankton in Lake Menjer, to measure the environmental factors that determine the diversity and abundance of phytoplankton, and to determine the environmental factors that most influence the diversity and abundance of phytoplankton in Menjer lake. This research uses a survey method in 5 areas (inlet, middle of the lake, aquaculture using floating net cage, tourism, and outlet). The parameters observed were the diversity and abundance of phytoplankton as well as the concentration of water quality parameters that determine the presence of phytoplankton (temperature, light penetration, pH, DO, TSS, TDS, PO4 and NO3). To determine the environmental factors that most influence the diversity and abundance of phytoplankton was analyzed using the Principal Component Analyses method with Past 4.10 software. The community structure of phytoplankton in Menjer Lake was dominated by Bacillariophyta (83%). Factors determining the development of phytoplankton in Lake Menjer are influenced most by light penetration and PO4 concentration. The input of PO4 concentration from the water catchment area must be controlled to prevent phytoplankton blooming in Lake Menjer.