Batch culture studies of phosphate uptake by non-immobilized (free) and immobilized (Ca-alginate-entrapped) Chlorella emersonii have shown that exponentially growing free cells remove phosphate from the medium five times more rapidly than cells in late stationary phase. Culture age is also shown to be an important factor in determining the uptake abilities of cells when in their immobilized state. When cells of different ages are immobilized in Ca-alginate and placed in a small-scale packed-bed reactors the effects of culture age are sufficient to produce significant differences in reactor performance lasting in the order of five days.