The relation of the operation of the hexose monophosphate pathway to cellular particles ofUstilago maydiswas investigated. Results obtained indicate that extracts from which all microscopically visible particles had been removed could oxidize glucose through the hexose monophosphate pathway. By tracer techniques it was found that all, or almost all, released CO2was obtained from the number 1 carbon of glucose. Spectrophotometric studies demonstrated the presence of glucose‐6‐phosphate dehydrogenase and 6‐phosphogluconic acid dehydrogenase. Radiochromatographic studies indicated the formation by mitochondria‐free extracts of glucose‐6‐phosphate, 6‐phosphogluconic acid and possibly ribose‐5‐phosphate. These extracts could metabolize glucose and 6‐phosphogluconate with oxygen uptake. It was concluded that in this fungus the energy‐producing and decarboxylating steps of the hexose monophosphate pathway could operate with the uptake of O2in the absence of mitochondria. The dehydrogenases present in the supernatant solutions can cooperate with an electron transfer sequence to O2in the supernatant. Presumptive evidence for the presence and action of glucokinase and phosphoriboisomerase was provided by the results of the aforementioned experiments. Isolated mitochondria were found by spectrophotometric means to contain cytochrome c oxidase and DPNH oxidase. Mitochondria were capable of using succinate as a substrate with oxygen uptake. Under some conditions isolated mitochondria showed a high endogenous rate of metabolism which could be halted by KCN. The use of isotopic tracer techniques indicated that extracts containing mitochondria could carry out the decarboxylation of citric acid via the tricarboxylic acid cycle.