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Harmful cyanobacterial blooms have become a global threat to human health and aquatic biota around the world. While the ecotoxicity of cyanobacterial toxins such as microcystins (MCs) has been studied extensively, the toxicity of non-toxin producing cyanobacteria has not been evaluated to the same extent. In this study, five strains of Microcystis wesenbergii were isolated from the Tri An reservoir and cultured under laboratory conditions. Microscopic observation was used for morphological identification. The MCs concentration was measured by high-performance liquid chromatography (HPLC). The microcrustacean (Daphnia magna) was exposed to different concentrations of crude extracts in a series of acute (48 h) and sub-chronic (15 day) toxicity experiments. The acute assay showed that crude extract from all isolated strains of M. wesenbergii generated toxic effects on D. magna, but no variant of MCs was detected in the cultures of M. wesenbergii. The 48 h EC50 values of crude extracts of M. wesenbergii on D. magna ranged from 307.2-491.5 mg dry weight (dw)/l. Sub-chronic exposure of D. magna to the crude extract of M. wesenbergii at concentrations of 1, 10, 40, and 120 mg dw/l resulted in a decline of survival rates with dose dependence. Both maturation and reproduction of parent D. magna were inhibited with increasing concentrations of crude extract. This finding indicated that crude extracts from non-MC-producing M. wesenbergii isolated from the Tri An reservoir had significant acute and chronic toxic effects on D. magna.