Abstract: The toxicity profile of coronatine (COR), a toxin produced by Pseudomonas bacteria using the Allium cepa test plant was determined with the help physiological analyzes including fresh weight, root number, root length and germination percentage; cytogenetic analyzes including chromosome aberration (CA), micronucleus frequency (MN), and mitotic index (MI); biochemical analyzes including superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA) and proline (PR) accumulation; and microscopic analyzes including changes in root anatomical structure. A. cepa bulbs were divided into four groups as one control (C) and three application. Group C bulbs were kept in cuvettes containing tap water for 168 hours (7 days), while treatment group bulbs were kept in cuvettes containing 1, 5, and 10 µM COR solutions. COR administration caused a decrease in all physiological parameters examined, a rise in the CA and MN frequency, also a diminish in MI compared to C group. COR promoted CAs such as irregular mitosis, nuclear peak, exposure of chromosome scaffold, chromosome losses, unequal seperation of chromosome, vagrant chromosomes and chromatid bridges. In addition, the mentioned application caused a dosege-bound enhancement in free PR, CAT, SOD and MDA contents according to C group. Moreover, 10 µM COR, the highest application dose, caused quite significant damages such as epidermis cell deformations, micronucleus in epidermis/cortex, accumulation of various chemicals in the cortex layer, thickening of the cortex cell wall, flattened cell nuclei, necrosis and unclear transmission tissue in root anatomical structure of the bulbs. In summary, it was concluded that COR is a chemical with inhibitive impacts and the Allium cepa testing is a utility bioindicator for following these impacts.