We have developed transgenic cucumber plants Cucumis sativus L. resistant to gray mold (Botrytis cinerea) by the expression of rice chitinase cDNA (Tabei et al. 1998). These transgenic plants were obtained by Agrobacterium-mediated transformation, and the introduced rice chitinase cDNA was driven by the CaMV 35S promoter. The biosafety assessment of the transformed cucumber plants was carefully performed to confirm that their characteristics are equivalent to those of non-transgenic plants except for harboring gray mold resistance. The biosafety assessment was carried out successively in full-containment greenhouse, semi-containment greenhouse, isolated field, and finally in ordinary field trials. The following items were compared between transgenic and non-transgenic cucumber plants: (1) morphological characteristics of plants and fruits; (2) reproductive characteristics, e.g. pollen morphology and fertility, longevity of the pollen, pollen dispersal by wind, and seed fertilities; (3) possibility of harmful influences on the environment due to the production of detrimental substances i.e. volatile compounds, plant parts and root secreta; (4) influence on soil microflora and; (5) presence of remaining Agrobacterium tumefaciens, which was used as a vector in the production of the transgenic cucumber. Throughout the biosafety assessment, we did not find any substantial differences except the expression of rice chitinase gene and resistance to gray mold between transgenic and non-transgenic cucumber plants.