Transmission electron microscopie (TEM) study of haemocytes in the Iarval Ornithodoros (Pavlovskyella) erraticus reveals three primary cell types: prohaemocytes, plasmatocytes and granulocytes. Prohaemocytes, with a high nucleoplasmic ratio, contain rough endoplasmic reticulum (RER), lysosomes, glycogen-like particles, lipid droplets and small granular inclusions, and probably serve as progenitors of the other haemolymph cell types. Plasmatocytes contain free ribosomes, mitochondria, RER, Golgi bodies, vacuoles and pseudopodia-like processes on the cell surface. Granulocytes, the predominent cell type in the haemolymph, contain numerous granules in addition to other cell organelles. Plasmatocytes and granulocytes are phagocytic and possibly also involved in other functions in the tick body. Nephrocytes are scattered in the haemolymph or attached to fat body strands. The plasma membrane forms deep invaginations whose openings are closed by diaphragms. Tubular elements, found in cells performing extensive endocytosis, connect the invaginated cell membrane with large coated vesicles, and probably transfer material from the haemolymph into the coated vesicles. These are specialized for internalization of proteins from the haemolymph. The cell cytoplasm also contains lysosomes, Golgi bodies, RER, mitochondria, and dense granules. These may develop by condensation of protein in the coated vesicles