Effect of the essential oil of Acorus calamus L. rhizomes, was studied on hemocytes of the tobacco armyworm, Spodoptera litura. The oil was administered in oral application at concentration of 500 and 1000 ppm to last instar larvae of S. litura and its effect on ultrastructure of hemocytes and hemogram was evaluated. The oil was administered in topical application at 250 microg dose to pupae to ascertain its effect on total and differential hemocyte counts. At both scanning (SEM) and transmission electron microscopic (TEM) levels, the major effect of oil treatment was observed on plasmatocytes (PLs) and granular hemocytes (GRs). SEM study revealed that the cytoplasmic projections of granular hemocytes were reduced, while the filopods of plasmatocytes remained unaffected. The vacuolization in the cytoplasm and degeneration of the organelles in both plasmatocytes and granular hemocytes was observed by TEM. However, no such deformities were observed in prohemocytes (PRs), spherulocytes (SPs), and oenocytoids (OEs). A concentration-dependent decrease has been observed in the larval body weight and hemolymph volume (HV), 24-72 h after treatment. In comparison to the controls, the maximum percentage growth inhibition (GI) was recorded to be 58.28 and 66.48, respectively at 500 and 1000 ppm after 72 h treatment. Similarly after 72 h treatment, the percentage reduction in hemolymph volume was 61.38 and 69.05, respectively at 500 and 1000 ppm. Total hemocyte count (THC), in larvae computed from five recorded hemocyte types viz. PRs, PLs, GRs, SPs and OEs, decreased only after 48-72 h of treatment. The maximum decrease in THC was recorded to be 29.15 and 49.05% at 500 and 1000 ppm, respectively, after 72 h of treatment. There was continuous decline in THC in pupae after 24-72 h treatment. DHC study revealed that both the concentrations of oil in 6th instar larvae of S. litura caused a decrease in PRs, PLs and SPs and increase in GRs and OEs after 24-72 h of treatment. Since A. calamus oil treatment causes the injury to both PLs and GRs and also affects the hemogram, it can be inferred that cellular defence reactions of S. litura are impaired.