The green alga haematococcus was tested for nitrate uptake in the presence of ammonium. Preference for the nitrate ion was controlled by initial pH of the medium and by age of the culture inoculum.Critical age and initiai pH forH.pluvialisFlotow, Cambridge U. strain 34/1aand forH.lacustris(Girod.) Rostaf., Indiana U. strain 294, were different. When inocula consisted of cells in log phase of growth, cultures ofH.pluvialispreferred ammonium in medium that was pH 7.0. Only a small percentage ofH.lacustriscultures preferred ammonium and only when the medium was as acid as pH 4.8. Increased age of the inoculum decreased sensitivity to pH of the culture medium.Nitrate uptake inH.lacustriswas less sensitive to external pH and seemed to be more exclusive than inH.pluvialis.