Arsenic is ubiquitous in the environment and potentially toxic to humans. Arsenate,thermodynamically theominant specie of arsenic in marine and estuarine surface waters,was shown to be taken up by the phosphate transportystems of phytop lankton and plants due to its similar structure to the phosphate. Considerable evidence has been foundhatMicrocystis luxuriously up take phosphate toform polyphosphate bodies in phosphate-rich environments. The cyanobac-erial sensitivity to arsenate has often been linked to the structural similarities of arsenate and phosphate,and intracellularolyphosphate was shown to be related to the sensitivity to arsenate. Therefore,the present research was intended to ex-lore the effects of arsenate on the growth and microcystin production ofMicrocystis aeruginosa FACHB905,which isolatedrom Dianchi Lake when only the extracellular phosphate concentration was changeable. The cells of M. aeruginosaACHB905 were cultivated in the phosphate-free BG-11 medium for 14 days in order to completely consume phosphatetored in the cyanobacterial cells. Then,these phosphate-starved cellswere inoculated in modified BG-11 media adjusteds following two cases: PO3 - was added at 1μM for the phosphate-limited medium,and PO3 - was absent as the phosphate-eprived medium. Arsenate as Na-HAsO4 was added to the culture media at concentrations from 10 - 8 to 10 - 4M. It wasorthwhile mentioning that the phosphate concentration used in this study (1μmol/L) was similar to that under naturalonditions. We measured the density of cultures,chlorophyll content and microcystin content of this cyanobacterium re-ponding to arsenate under both the phosphate regimes. This study showed that the extracellular phosphate concentrationad no reference to the threshold doses (- 10-7mol/L) ofM. aeruginosa FACHB905 to arsenate. However,the IC50 val-e under phosphate limitation was 10--.79mol/L and three magnitudes greater than that under phosphate deprivation. Thepparent association constant of arsenate to the cyanobacterium under phosphate limitation wasmuch lower than that underhosphate deprivation. Thus,it presumed that extracellular phosphate had the key role on protecting cyanobacterial cellsrom arsenate. Otherwise,arsenate did not affect the chlorophyll content per cell,but had dosage effect on the cellularmi-rocystin content. Arsenate,higher than 10-7mol/L,could promote the cellularmicrocystin content per cell under phos-hate limitation,while the microcystin content of all arsenate treatments was stimulated about 78% of that of the controlnder phosphate deprivation. The synergistic effect of arsenate and microcystin production ofM. aeruginosa FACHB905 isf definite significance for complete understanding the microcystin production in the blooms in Dianchi Lake.