Raphidiopsis raciborskii is a globally distributed bloom-forming cyanobacterium and produces toxin cylindrospermopsin (CYN), which results in public and environmental health consequences. piece of evidence To understand their blooming dynamics and to identify the environmental factors that determine their success in the tropical reservoir, a monthly monitoring program was conducted from May 2019 to April 2020. Variations in phytoplankton communities, environmental parameters (temperature, pH, dissolved oxygen, turbidity, dissolved inorganic nutrients) and CYN were simultaneously determined from a total of 36 samples in the Ea Nhai reservoir in Vietnam. The phytoplankton is composed of a dominant cyanobacteria community formed mainly by filamentous R. raciborskii with the biovolume of up to 66.8 mm³ L^-1. PCA and Pearson correlation results showed the biovolume and variation of R. raciborskii in the Ea Nhai reservoir were mainly related to temperature and nutrients (N-NH₄, P-PO₄, TP, TN). Analysis of the toxins by ELISA demonstrated the presence of CYN in all water samples with an average value of 1.24 μg L^-1 and it was significantly correlated with the biovolume of the invasive species R. raciborskii. The results of the HPLC analysis confirmed the production of CYN in nine isolated Raphidiopsis strains with concentrations ranging from 0.054 to 0.584 μg g^-1DW. The relatively high biovolume of R. raciborskii that is associated with the level of CYN in the water and the capacity to produce CYN of the isolated R. raciborskii strains represent a potential threat to the public health and the life of domestic and wild animals in the reservoir catchment.