Physiological effects of 2,4-D on the growth of Chlorella ellipsoidea were investigated culturing the alage in the MN4 media containing 0. $10^{-4}/M$ and $4< imes}10^{-4}M$ 2,4-D. During 6 days culture were taken to analysis with respect to overall growth, photosynthesis, respiration and chemical composition. Results obtained from the experiment were as follows : 1) The growth of chlorella was increased at $10^{-4}M$ and decreased at $4{ imes}10^{-4}M$ of 2,4-D concentrations 2) At $10^{-4}M$ pf 2,4-D cpncentration, the activity of photosynthesis enhanced relative to contro. while at $4{ imes}10^{-4}M$ it was not changed. In both concentrations, however, the rate of respiration was down from the control. 3) At $10^{-4}M$ 2,4-D, the concentration of carbondrate metabolites was not changed relative to control, while significant increase in the concentrations of proteins and nucleic acids was observed. On the other hand at $4{ imes}10^{-4}M$ of 2,4-D concentrations, all the metabolites including carbohydrates, proteins and nucleic acids were descreased. 4) It is concluded that 2,4-D at $10^{-4}M$ concentration accelerates the growth of chlorella by promoting the activities of photosynthesis and biosynthesis of proteins and nucleic acids.