The arsenate (As(Ⅴ)) biotransformation by Microcystis aeruginosa in a medium with differentconcentrations of nitrogen (N) and phosphorus (P) has been studied under laboratoryconditions.When 15 μg/L As(Ⅴ) was added,N and P in the medium showed effectiveregulation on arsenic (As) metabolism in M.aeruginosa,resulting in significant differences inthe algal growth among different N and P treatments.Under 0.2 mg/L P treatment,increases inN concentration (4-20 mg/L) significantly stimulated the cell growth and therefore indirectlyenhanced the production of dimethylarsinic acid (DMA),the main As metabolite,accountingfor 71％-79％ of the total As in the medium.Meanwhile,10-20 mg/L N treatments acceleratedthe ability of As metabolization by M.aeruginosa,leading to higher contents of DMA per cell.However,As(Ⅴ) uptake by M.aeruginosa was significantly impeded by 0.5-1.0 mg/L P treatment,resulting in smaller rates of As transformation in M.aeruginosa as well as lower contents of Asmetabolites in the medium.Our data demonstrated that As(Ⅴ) transformation by M.aeruginosawas significantly accelerated by increasing N levels,while it was inhibited by increasing Plevels.Overall,both P and N play key roles in As(Ⅴ) biotransformation processes.