The effect of sulfate and phosphate deprivation on cell growth and cylindrospermopsin level was studied in Aphanizomenon ovalisporum ILC-164. Sulfate starvation induced a characteristic reduction of cylindrospermopsin pool size on the basis of cell number and unit of dry mass of culture. Phosphorous starvation of A. ovalisporum cultures induced a lesser reduction of cylindrospermopsin pool size. This divergence in the pool size of cylindrospermopsin may be the consequence of different growth rate. To show the metabolic changes concomitant with reduction of cylindrospermopsin pool size were obtained by measurement of ATP sulfurylase and alkaline phosphatase activity. The present study is the first concerning the cylindrospermopsin content under sulfate starvation and discusses it in relation to phosphorous starvation.