Cockroach species Periplaneta americana and Blattella germanica potentially survive in locations close to human activity. Besides spoiling food material, cockroaches also transfer pathogens of different diseases among human beings. Since the insecticides have been used extensively to control cockroaches, information on their insecticide susceptibility and toxicity at the cellular level may be crucial. In the study, deltamethrin toxicity as well as the deltamethrin-mediated cytomorphological changes in the brain, ovary and midgut of the two important cockroach species have been assessed. Different concentrations [0.00025% (0.0025 mg/ml), 0.0025% (0.025 mg/ml), 0.025% (0.25 mg/ml), 0.25% (2.5 mg/ml), 0.5% (5 mg/ml), 1% (10 mg/ml)] of deltamethrin in acetone were used to expose test species in WHO bottle assay. Knockdown was recorded after 5 min interval while delayed mortality was observed after 24 h. Brain, ovary and gut were dissected post 1 h exposure and 24 h holding (for 0.25, 0.5 and 1% concentration), and tissues were processed for microscopic analysis. Deltamethrin exposed cockroaches and dissected tissues were used to estimate deltamethrin using HPLC. At 0.00025% (lowest concentration), the percentage knock-down observed was 66.7% for P. americana and 80% B. germanica respectively (R2 = 0.78; p = 0.0001) in 1 h. KDT 50 value was found to be 8.7 min (95% CI: 7.3–10.2), while KDT 99 was 20.7 min (95% CI: 16.0–35.7) in P. americana at 1% concentration. Whereas, the KDT 50 and KDT 99 values for B. germanica were 7.4 min (95% CI: 5.4–9.1) and 27.4 min (95% CI: 18.2–80.0) at a similar concentration. LD50 and LD 95 values (for 60 min standard exposure) were 0.0006% (95% CI: 0.00–0.001) and 0.034% (95% CI: 0.013–0.49) respectively for P. americana, while these values were 0.0005 (95% CI: 0.00–0.001) and 0.04 (95% CI: 0.01–0.23) for B. germanica. Exposure to 1% deltamethrin induced a considerable toxic effect in the epithelial cells in the midgut. HPLC estimated 0.21 ± 0.05 mg (95% CI: 0.18–0.25; CoV 23.9%) deltamethrin in P. americana post 1% exposure. Even short term exposure to a low concentration of synthetic pyrethroid deltamethrin displayed immediate knockdown and delayed mortality in both the test species. Considerable histological damage was observed in both the insects at 1% exposure. In India, resistance to deltamethrin may have been reported among different insects due to its extensive use. However, the formulations such as insecticide paints, attractant baits etc. developed using deltamethrin as an active ingredient could be useful in cockroach control operations. [ABSTRACT FROM AUTHOR]