The utilization of glycollate as a substrate for photoheterotrophic growth by a strain of Chlorella pyrenoidosa has been demonstrated. Glycollate stimulated growth of this alga in basal medium over the pH range 4.0 to 8.0 in the light, but did not support growth in the dark. Stimulation of growth in the light occurred over a wide range of glycollate concentrations and was optimal at 30 mM. Enzyme and inhibitor experiments suggested that the synthesis of cell constituents during growth on glycollate is achieved by the same pathway which operates during the metabolism of exogenous glycollate by autotrophically-grown cells. For algae to metabolize and grow on exogenous glycollate the cells must be readily permeable to this compound. When the cells readily take up exogenous glycollate, the level of activity of enzymes in the cell, in particular glycollate:DCPIP oxidoreductase, may regulate the over-all rate of glycollate metabolism.