The distribution of radioactivity after feeding labeled glucose, as well as the change of the poolsize of intermediates, in Chlorella poisoned with monoiodacetic acid (MIA), showed that triosephosphate dehydrogenase is the most sensitive Enzyme in respiration. MIA leads to a large accumulation of fructose diphosphate (FDP) and a large decrease in phosphoglyceric acid (PGA). Also, the breakdown of glucose via 6-phospho-gluconate is inhibited by MIA, as shown by the small change of the C1/C6 ratio. FDP piles up during MIA poisoning in the dark and is quickly transformed to ribulose diphosphate (RuDP), in the light; subsequently, RuDP is transformed to PGA and its derivatives (pyruvate and other acids) by carboxylation and hydrolysis of the ketoacid. Thus, a third pathway of glucose breakdown, by passing glycolysis and the 6-phosphogluconate pathway, is induced by MIA in the light. The photosynthetic CO2 assimilation of Chlorella is about 100 fold more sensitive toward MIA than respiration. By feeding labeled CO2 and determining the concentration of the intermediates formed during MIA poisoning in the light, it is found that, in contrast to the behavior of broken chloroplasts, the reduction of the fixed CO2 is the most sensitive reaction of photosynthesis in Chlorella, rather than the formation of RuDP. It is assumed that the energy-rich phosphate formed in the light may be used for a relatively MIA-insensitive reaction leading to the formation of RuDP, and that the carboxylated RuDP is directly reduced to sugar by a highly ΜΙΑ-sensitive reaction. Hydrolysis to PGA occurs only if the reduction of the ketoacid proceeds too slowly, i. g., during MIA-inhibition.