ABSTRACTDunaliella tertiolecta(Butcher) was grown in chemostat culture over a wide range of ammonium limited growth rates. The addition of ammonium caused a rapid temporary suppression of photosynthetic carbon fixation. The magnitude of ammonium induced photosynthetic suppression increased with the severity of ammonium limitation. Cells growing at rates greater than ca. 80%μmaxexhibited no photosynthetic suppression in response to additions of the limiting nutrient. The duration of photosynthetic suppression was related to the concentration of added ammonium. Immediately following the suppression, photosynthesis was enhanced with respect to the controls. The eventual degree of enhancement increased with the concentration of added ammonium. Steady‐state cellular chlorophyll concentrations, photosynthetic rates, and assimilation numbers are report