The green alga, Chlorella vulgaris was immobilized in Ca-alginate beads at two stocking cell densities and grew in primarily treated domestic effluents. It was found that the cellular metabolic activities of this alga were retained after immobilization. The growth (in terms of cell counts) and photosynthetic rates of cells immobilized in beads of low stocking density were greater than those in beads of high density. Significant reduction of wastewater NH4+-N and PO43−-P was recorded especially in reactors containing algal beads of high density. These results suggested that immobilized Chlorella cells can be used as secondary treatment process for domestic sewage.