SYNOPSIS.Trophozoites ofGiardia murisparasitic in hamsters were fixedin situat intervals of 1–3 hr after injection of horse ferritin into the small intestinal lumen. Concentrations of ferritin molecules are localized within vacuoles beneath the plasmalemma along the dorsal and ventral surfaces ofGiardia.Limiting membranes of these vacuoles are identical with the plasmalemma. It is suggested that the vacuoles function in sequestration of molecules from the intestinal lumen by invagination of the cell membran