Digestion of blood by the soft tick O. moubata is investigated by physico-chemical methods in the extracellular compartment of the midgut lumen and by electron microscopy in the intracellular compartment of the midgut cells. Undigested blood may be stored (in form of orthorhombic crystals, in the case of guinea-pig blood) over months in the midgut lumen, although a drop of the pH is observed there sorne days after bloodmeal. Digestion is intracellular indeed and corresponds to a classical lysosome concept, leading to heterophagosomes by the fusion of phagocytosed material or pinocytotic vesicles with intracellular enzymes produced by the GERL complex. Ultrastructural changes related to the digestive cycle are related in a following paper (GRANDJEAN, 1983).