The effect of arsenate on cells of a marine cyanobacterium, Phormidiurn sp. preliminarily starved for phosphate for a week was studied. Cells were harvested and cultured in artificial seawater containing various concentrations of arsenate and phosphate. Arsenate at concentrations above 30 mg As dmP3 inhibited biosynthesis in the cells and consequently, growth when incubated without phosphate in the medium. On the contrary, phosphate at 50 pmol dm-3 was sufficient for apparently complete cancellation of the inhibitory. effects of arsenate at concentrations up to 150 mg As dm-3. Study of the carbohydrate metabolism revealed an intense inhibition by arsenate on turnover of carbohydrate to other cell components in the phosphate-depleted cells. This resulted in a color change of the cells from blue-green to yellowish. The synthesis of carbohydrate itself was also inhibited by arsenate. Arsenate incorporation into cells was clearly inhibited by phosphate in the medium, suggesting that arsenate competes with phosphate for entry into cells. In addition, arsenate incorporated in cells could not inhibit the incorporation of phosphate and subsequent growth of cells on phosphate. These observations indicate that arsenate can act as a poisonous substitute for phosphate in the cells but, once incorporated into the phosphate-replete cells, it no longer has an inhibitory effect. The inhibitory effects of arsenate seem to be mainly related to ATP synthesis in the photosynthetic system.