We introduced a modified delta-endotoxin gene, modified cry1Ab (mcbt) of Bacillus thuringiensis, which displays a specific biological activity against lepidopteran insects into chrysanthemum [Dendranthema × grandiflorum (Ramat.) Kitamura]. The chrysanthemum cultivar ‘Shuho-no-chikara’ was transformed using a disarmed strain of Agrobacterium tumefaciens, EHA101, carrying a binary vector, pIG121mcbt that harboured a mcbt gene encoding an insecticidal crystal protein (ICP) fragment of B. thuringiensis var. kurstaki HD-1. Leaf discs were co-cultured with Agrobacterium and thereafter cultured on the callus induction medium containing G418. A total of 317 shoots were regenerated from 3,618 leaf discs on the regeneration medium (8.8 %). The mcbt gene was detected in all the regenerated plantlets by Southern blot analysis. The accumulation of Cry1Ab ICP in 20 transformed lines, selected at random, was confirmed by Western blot analysis. The level of accumulation of Cry1Ab ICP ranged from 10.5 ng to 80 ng per 50 μg total soluble protein (from 0.021 to 0.16 % of the total protein). Insect bioassay was conducted using tobacco budworm (Helicoverpa armigera) larvae and all the larvae of H. armigera tested died during the first instar on the leaves of the mcbt-transformed lines whose expression level of Cry1Ab ICP exceeded 47.6 ng per 50 μg compared to the wild type cry1Ab-transformed lines. Significantly higher feeding inhibition and/or growth inhibition of the insects was observed, compared to those on the non-transformed control plants.