The phosphate concn. was varied to ascertain if the prodn. of arsenosugars by Fucus could be stimulated by a deficit of phosphate, which would encourage the uptake of arsenate, or by an excess of phosphate which would possibly increase metab. The results indicate that with a low phosphate concn. arsenate uptake is enhanced and as a result the concn. of the usual metabolites arsenite and dimethylarsenic acid are increased in the Fucus and the seawater.