To assess the ecological impact of arsenic pollution during cyanobacterial blooms, As speciation and cyanobacterial growth in phosphate-modified Microcystis aeruginosa cultures treated with arsenate were investigated under laboratory conditions. Marked growth inhibition was observed when arsenate was added. The inhibition effect of 1 µM arsenate was lower than that of 10 µM arsenate. Increasing phosphate supply (0-175 µM) in the medium decreased the inhibition of As. In the medium, arsenate, arsenite, and dimethylarsenicals (DMA) occurred under phosphate-deprivation (0 µM) and phosphate-excess (175 µM) conditions. However, only arsenate and DMA were detected under phosphate-limited (1 µM) and phosphate-rich (10 µM) conditions. Moreover, arsenite and DMA concentrations had significantly positive correlation with the biomass production of M. aeruginosa in the existence of phosphate. Arsenic speciation was more significantly affected by phosphate levels than arsenate concentrations. Recovery of total As content in M. aeruginosa culture medium increased with the increasing phosphate supply. The duration of arsenate contamination in the culture of M. aeruginosa had no influence on the variation of As species but affected the concentration of them in the medium under the phosphate-excess condition. This demonstrated that the effect of M. aeruginosa on As speciation was not related to the duration of As contamination under the phosphate-excess condition.