The black cutworm Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), is globally distributed. It is a polyphagous insect attacking nearly all vegetables and many economic field crops in the world. The objective of the current study was to investigate the efficacy of two Entomopathogenic nematodes, Steinernema carpocapsae and Heterorhabditis bacteriophora on the main body metabolites in haemolymph of the infected larvae. The newly moulted penultimate (5th) instar larvae of A. ipsilon had been infected with LC₅₀ values of S. carpocapsae and H. bacteriophora (21 IJs/ml and 62 IJs/ml, respectively) and the influenced contents of the metabolites in haemolymph of last (6^th) instar larvae were determined at three-time intervals of the instar, 6, 24 & 48 hr, respectively. The most important results could be summarized as follows. The protein content in the infected larvae was predominantly reduced by both nematode species. Moreover, it was tremendously reduced at the last time interval of exposure (21.94 & 26.45% protein reductions, by S. carpocapsae and H. bacteriophora, respectively). Thus, H. bacteriophora exhibited stronger reducing potency than S. carpocapsae. Also, the lipid content in haemolymph of EPN-infected larvae was gradually reduced with the time intervals. The greatest reduction of lipids was determined at 48 hr post-infection (26.55 & 21.73% lipid reduction, by S. carpocapsae and H. bacteriophora, respectively). Thus, S. carpocapsae exerted greater reducing action than H. bacteriophora. The carbohydrate content was predominantly reduced in haemolymph of infected larvae. S. carpocapsae exhibited a higher reducing effect on carbohydrate content than H. bacteriophora, at 48 hr of the last instar. Almost, S. carpocapsae had greater reducing potency against the main body metabolites in haemolymph of the last instar larvae of A. ipsilon, leading to drastically disrupted intermediary metabolism. Therefore, S. carpocapsae can be applied as an effective part of the Integrated Pest Management program against this serious pest.