Post-transcriptional gene silencing, such as antisense suppression, sense suppression (or cosuppression), and RNAi, is often used to down-regulate a target gene in transgenic plants. Novel flower color is industrially important; furthermore, flower color is a convenient tool to monitor the stability of such silencing. Previously, we obtained white torenia through sense suppression of chalcone synthase or dihydroflavonol 4-reductase (Suzuki et al. 2000). However, their phenotypes were not stable. In this study, we suppressed the anthocyanidin synthase gene using three methods in transgenic torenia. About half of the transgenic torenia plants gave white flowers by RNAi suppression of the gene, while antisense and sense suppression yielded a few and no white flowers, respectively. The white flower color obtained by RNAi has been stable for three years in a greenhouse. This study shows the usefulness of RNAi to suppress a target gene.