We examined the effect of pretreatment on the frequency of flower-color mutants induced by ion beams. We found that petunia seedlings treated with 3% sucrose from 8 days after sowing accumulated significant amount of pigments within 4 days compared to non-treated control seedlings. The petunia seedlings treated with sucrose were exposed to 320-MeV carbon ions. The sucrose treatment did not affect the survival rate and seed fertility of the M₁ plants. In the M₂ lines obtained by self-pollination of individual M₁ plants, chlorophyll mutants were obtained in both treated and non-treated groups with a similar frequency. Flower-color mutants that included magenta, purple, light pink and white were obtained from the original violet color. The frequency of flower-color mutants was significantly higher in the sucrose-treated group than in the non-treated group. These results suggest that sucrose pretreatment specifically increases the frequency of flower-color mutation following ion beam irradiation.