Effect pf gibberellic acid (GA) on the growth rate, respiratory activity and solute uptake of Chlorella cells were measured and their correlation were discussed. Growth rate and respiratory activity of the algal cells are enhanced considerably by very samll amount (50 ppm) of GA treatment although they are suppressed by relatively higher concentration more than 100 ppm. Phosphate uptake of the algal cells, however, decreased even though lower concentration of GA is applied. Thereforem it is inferred that the growth enhancement of the algae by GA is not due to the increase of the permeability of the algal cells but expansion growth owing to the increase of osmotic pressure caused by the increase of hydrolase activity of the algae.