Abstract. Sulastri, Henny C, Nomosatryo S. 2019. Phytoplankton diversity and trophic status of Lake Maninjau, West Sumatra, Indonesia. Pros Sem Nas Masy Biodiv Indon 5: 242-250. Microcystis blooms have been recorded in Lake Maninjau in 2000, October 2011 and recently on April 2018. Microcystis bloom was characterized by green color enhancement in the column water due to high concentration of chlorophyll-a reach to > 100 µg.L-1. The occurrence of phytoplankton bloom in this lake was not recorded all the year but Microcystis was disappeared and the water is clear again in a certain period. This study was aimed to reveal phytoplankton diversity and relation to trophic status in Lake Maninjau. Phytoplankton diversity and water quality include water transparency, temperature, DO, pH, conductivity, and nutrient were observed in 2009, 2014, 2015, 2016 dan 2018. Phytoplankton sample and water quality parameters were measured in the euphotic zone at nine stations. Three variable chlorophyll-a, Secchi depth (transparency) and total phosphorus concentration used to calculate the trophic state index. There are 121 species of phytoplankton belong to phylum Chlorophyta (74 species), Cyanophyta (23 species), Pyrrhophyta (7 species) dan Euglenophyta (4 species). The trophic status in Lake Maninjau showed variability temporally; it was mesotrophic, meso-eutrophic, eutrophic and hiper-eutrophic. Hypereutrophic status was characterized by low phytoplankton diversity (27 species), high chlorophyll-a, nutrient concentration, and Microcystis aeruginosa abundance (4180.932 x 103 individual.L-1). Mesotrophic was characterized by high phytoplankton diversity (63 species) belong to phylum Chlorophyta especially family Desmidiaceae, Oocyataceae, Scenedesmusceae, and low nutrient concentration. The higher trophic status is more decrease phytoplankton diversity.