The aim of this study was to explore phytotoxicity and oxidative stress as a mechanism of bisphenol A (BPA) toxicity in onion roots (Allium cepa L.). Roots of onion bulbs (n = 10) were exposed to either distilled water (negative control) or BPA solutions (concentration range 1-50 mg/l) for three days. After a three-day exposure, the fresh root weight of each bulb was weighed, and in 5% TCA a homogenate of onion roots was prepared (100 mg of tissue in 0.5 ml of 5% TCA). In the supernatant of prepared homogenate oxidative stress parameters: the content of reduced glutathione (GSH) and malondialdehyde (MDA) was determined, and in the homogenate’s precipitate the content of protein carbonyls was assessed. The GSH content was determined based on the Ellman method (1959), MDA by using the thiobarbiturate test and the content of protein carbonyls was determined according to Levin et al. (1990), by derivatization of the proteins’ carbonyl group with DNPH. The obtained results were statistically analysed by student’s t-test (Excel, MS Office).The fresh tissue weight of onion roots treated with BPA (in concentration range 1-50 mg/l) was lower compared to the negative control, and a significant decrease in roots fresh weight was recorded already after exposure to BPA in concentration of 5 mg/l. The decrease in fresh tissue weight with increasing BPA concentration indicates that BPA had phytotoxic effect on onion roots. The exposure to BPA in concentration of 1 mg/l significantly decreased the content of GSH compared to the control. A significant increase compared to the negative control in the content of protein carbonyls was recorded after exposure to BPA in concentration of 10 mg/l, and MDA after exposure to BPA in concentration of 25 mg/l. A decrease in the level of antioxidants (GSH) and an increase in the level of lipid and protein oxidative damage parameters (MDA and protein carbonyls) indicate that BPA caused oxidative stress in the model organism. The obtained results imply that BPA caused oxidative damage to cellular structures, thus can be concluded that oxidative stress is one of the mechanisms of BPA toxicity.