The black cutworm, Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), is widely distributed in the world. It is a polyphagous insect attacking nearly all vegetables and many economic field crops causing a great economic loss. The current research was conducted to evaluate the corruptive effects of entomopathogenic nematodes (EPNs), Steinernema carpocapsae (Weiser) and Heterorhabditis bacteriophora (Poinar) on the hemocyte profile of the A. ipsilon larvae and investigate the interaction between their immune defences and EPN-bacterium complex immunosuppression. For achieving this purpose, the newly moulted 5^th instar larvae were infected with LC₅₀ concentrations of S. carpocapsae and H. bacteriophora (21 & 62 IJs/ml, respectively) and the immune reactions were described at certain time intervals post-infection and recorded by photomicrographs. The most important results could be summarized as follows. Five main types of normal circulating hemocytes had been identified, viz., prohemocytes (PRs), plasmatocytes (PLs), granulocytes (GRs), spherulocytes (SPs) and oenocytoids (OEs). The immune responses of A. ipsilon larvae to EPNs were described as encapsulation, nodulation and phagocytosis. On the other hand, the EPN-bacterium complex exhibited different features of suppression on the immune system of A. ipsilon larvae. The major features were considerable deformations of the host hemocytes. At 24 to 48 hrs after infection with S. carpocapsae and H. bacteriophora, toxins produced by their symbiotic bacteria, Xenorhabdus nematophilus and Photorahbdus bacteriophora, respectively, attack A. ipsilon hemocytes leading to death. Because the insects have only innate immune responses against invading pathogenic microbes by immunocytes, the destruction of these hemocytes constitutes a promising approach to the biocontrol of insect pests. Therefore, the tested EPNs S. carpocapsae and H. bacteriophora, can be used in the integrated pest management of the dangerous insect, A. ipsilon.