Data from the present study indicate that the neonicotinoid insecticides Nuprid 200 SL (with the active substance imidacloprid) and Calypso 480 SC (with the active substance thiacloprid) reduce the mitotic activity by inhibiting cell division and increasing chromosomal aberrations in the cells of the root apical meristem of Allium cepa L. The tested solutions of pesticides cause a wide range of anomalies associated with disorders in forming the cell's division apparatus and the integrity of chromosomes. The comparative analysis of their mutagenic action shows a higher genotoxic potential of Calypso 480 SC. The established indices of chromosomal abnormalities for Calypso 480 SC and Nuprid 200 SL are 1.07 ± 0.38% and 0.70 ± 0.41% respectively. The mitosodepressant effect and a large number of chromosomal aberrations and mitotic abnormalities in Allium cepa cells when treated with Nuprid 200 SL and Calypso 480 SC are evidence of their high toxic potential and the significant risk of environmental pollution by their use in agriculture. Parts of this work was published in abstract form and presented as an oral presentation at the IV International Agricultural, Biological & Life Science conference, Edirne, Turkey, 2022 August 29-31, [1] and as a poster presentation at the 5th Balkan Scientific Conference on Biology, Plovdiv, Bulgaria, 2021 April 15-16 [2].