Haemocytes of Ixodes scapularis were characterized on the basis of their ultrastructure by transmission electron microscopy of thin sections. Three types of haemocytes were identified: prohaemocytes, plasmatocytes, and granulocytes. Prohaemocytes are undifferentiated cells containing very little cytoplasm (high nucelo-cytoplasmic ratio). Plasmatocytes are rich in free ribosomes, mitochondria, rough endoplasmic reticulum, and have numerous peripheral vacuoles. Granulocytes are polymorphic cells containing granular inclusions and filopodia on the surface. Granulocytes are divided into two morphological types, type I and II, based on the structure of the granules.