In the present study, the protective effect of Royal Jelly (RJ) on genotoxicity and lipid peroxidation, induced by petroleum wastewater, in Allium cepa L. root-tip cells was investigated. For this purpose, we used the malondialdehyde (MDA) level, mitotic index (MI), frequency of micronucleus (MN) and chromosomal aberrations (CAs) as indicators of genotoxicity and lipid peroxidation, and correlated these data with statistical parameters. In additional to the genotoxic analysis, we examined changes in the root anatomy of A. cepa seeds treated with the wastewater. Heavy metal concentrations in the wastewater were measured by atomic absorption spectrophotometry. The seeds were divided into six groups as control, wastewater and RJ treatment groups. They were treated with the wastewater alone, RJ alone (25 and 50 mu m doses) and RJ + wastewater for 10 consecutive days. As a result, the mean concentrations of heavy metals in the wastewater were observed to be in the order: Pb Fe Al Ni Cu Zn Cr Cd. The results showed that there was a significant alteration in MI and in the frequency of MN and CAs in the seeds exposed to the wastewater when compared with the controls. The wastewater exposure resulted in a significant increase in CAs and MN formation (P 0.05). The wastewater also caused a decrease in MI (P 0.05). Additionally, there was a significant increase in the MDA levels of the roots exposed to the wastewater (P 0.05). Heavy metals in the petroleum wastewater significantly increased the MDA production, indicating lipid peroxidation. Moreover, light micrographs showed anatomical damages such as an accumulation of chemical compounds in cortex parenchyma, cell death, an unusual form of cell nucleus and unclear vascular tissue. However, the RJ treatment caused amelioration in the indices of lipid peroxidation and MI, and in the frequency of CAs and MN, when compared with the group treated with petroleum wastewater alone (P 0.05). Also, the RJ application caused the recuperation of anatomical structural damages induced by the petroleum wastewater. Each dose of RJ provided protection against the wastewater toxicity, and the strongest protective effect was observed at dose of 50 mu m. In vivo results showed that RJ is a potential protector against toxicity induced by petroleum wastewater, and its protective role is dose-dependent.