A histological and histochemical study of ingested food material, energy stores and enzymes in the monogenean Pseudodactylogyrus anguillae, parasitizing the gills of the European eel (Anguilla anguilla) is presented. It was found that mucus, epithelial cells and blood from the gills were ingested. Glycogen deposits were small and primarily located in the parenchyma and to a minor extent in the vitellariae. Numerous globules of neutral lipids were found in the vitellariae. A marked esterase activity was found in the gut and a less marked activity in the vitellariae. Acid phosphatase activity was found throughout the body whereas alkaline phosphatase and leucine-amino-peptidase were not detected. Marked activity of succinate dehydrogenase and NADH-diaphorase was found in all cells, indicating a predominantly aerobic metabolism in this monogenean.