The functional carboxyl group of chlorophyll in Chlorella can be split off by sodium fluoride without killing or injuring the cells. For each mole of chlorophyll 1 mole of CO2 is given off and the more rapidly the CO2 the greater the concentration of the fluoride. Fluoride causes no evolution of CO2 from bleached cells and in cells in which the content of chlorophyll varies, the amount of CO2 split off varies with the chlorophyll content. This shows that the CO2 that can be split off by fluoride comes from the chlorophyll. The liberation of CO2 by fluoride is almost completely inhibited by 10-4 N HCN. No CO2 is split off by fluoride from suspensions of Chlorella heated to 65[degree] for 5 min. or from dried frozen suspensions. The liberation, accordingly, is closely associated with processes dependent on life. Evidence is presented to show that the carboxyl group (11) of chlorophyll can not be the functional one. Thus, chlorophyll in life must have another carboxyl group which previously has not been included in the chlorophyll formula and which, probably, is attached to C-atom (10) along with carboxyl group (11). The reactions concerned in the chemical mechanism of CO2 assimilation are given.