Abstract 1. 1. Activity levels and the intracellular distribution of various representative enzymes of energy-supplying metabolism were determined in Dicrocoelium dendriticum and the results compared with those obtained for the corresponding enzymes in rat liver. 2. 2. Phosphofructokinase and fructose-1,6-diphosphatase have been partially purified from D. dendriticum . Some characteristics of these enzymes were found to be of significance in the control of glucose degradation and biosynthesis. 3. 3. The presence of the pentose cycle enzymes glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase was established, and their functional activities were indicated by the higher degree of appearance of label in the excreted CO 2 derived from 1- 14 C-glucose than that produced from glucose labelled in position 6. 4. 4. D. dendriticum incorporates 14 CO 2 via an active phosphoenolpyruvate carboxykinase. The activity of this enzyme is confined exclusively to the extramitochondrial compartment of the cell. 5. 5. Malate decarboxylation activities are associated with both the extra- and intramitochondrial cell compartment of the parasitic tissue. Within the mitochondria malic enzyme activity was greater in the presence of NAD + than in the presence of NADP + . 6. 6. The complete complement of enzymes required for Krebs cycle activity is available in D. dendriticum , and the specific activities are, with the exception of α-ketoglutarate dehydrogenase, similar to those obtained from rat liver. In addition, mitochondrial membranes of the parasite were capable of reducing fumarate in the presence of NADH. Regardless of the presence of all cycle activities in the parasites, the relatively low level of α-ketoglutarate dehydrogenase complex as well as the small amounts of 14 CO 2 liberated from 6- 14 C-glucose suggest that the cycle with respect to its significance in terminal oxidation may be functioning at most to a limited extent. 7. 7. Both extra- and intramitochondrial α-glycerolphosphate dehydrogenases were found in D. dendriticum . Although substantial amounts of the mitochondrial enzyme could be detected, the extremely low activity of NAD + -linked α-glycerolphosphate dehydrogenase indicates that the glycerolphosphate cycle may be of minor importance. 8. 8. As a result of these studies a tentative diagram of the fermentation pathway in D. dendriticum is proposed.