Chlorogonium elongatum D angeard , an acetate alga, is characterized by its high productivity. Its growth constants in both, autotrophic and heterotrophic cultures exceed those of common strains of Chlorella or Euglena by a factor of 3–4. This seems remarkable in view of the large size of the cells (16 × 9μm). In cultures transferred from autotrophic growth to dark growth on acetate the plastids maintain nearly their original size; their internal membranes, however, are reduced to some single thylakoids or vesicles. Correspondingly, chlorophyll and activity of carboxydismutase (RuDPCO) become reduced to 5 and 10 % of that of the autotrophic culture. NADP-dependent glyceraldehydephosphate dehydrogenase (GAPDH) cannot be detected in heterotrophic cells; NAD-dependent GAPDH could be found in slight amounts. While isocitratase activity increases to a nine-fold amount by acetate growth, phosphoenolpyruvate carboxylase at the same time is reduced to about 50 %. When the cells had consumed the acetate in the dark, chlorophyll as well as RuDPCO and NAD-GAPDH begin to increase. NADP‐GAPDH is formed within a very short time; its activity surpasses that of the NAD-enzyme by 300 % after 8 hours Starting from dark cells, complete autotrophy is attained in the light within 8 hours. In this case, both types of GAPDH are produced, the NAD type, however, to a smaller extent