Hemocytes of the fifth instar European borer, Ostrinia nubilalis Hubner (Lepidoptera: Pyralidae) were examined in stained and unstained preparations. Prohemocytes, plasmatocytes, granular hemocyter, and oenocytoids, and spherule cells were found in this stage, The prohemocytes are characterized by a scant intensely basophilic cytoplasm. The highly pleomorphic plasiratocytes are characterized by a punctate nucleus and production of cytoplasmic extensions in vitro. Granular hemocytes are characterized by a small nucleus, accumulation of lipid droplets, and the production of extremely fine cytoplasmic extensions in living preparations. The oenocytoid is characterized by a small eccentric nucleus in a large amount of homogeneous basophilic cytoplasm. The spherule cell is characterized by large spherules within the cytoplasm which may tend to mask the nucleus.Treatment of larvae with per os doses of Bacillus thuringiensis did not affect the differential hemocyte count. However, there did appear to be an increase in the number of degenerating cells in Bacillus thuringiensis treated groups.Ball State UniversityMuncie, IN 47306