Prevention of precocious bolting, i.e. stem elongation accompanied by flowering, in Japanese radish (Raphanus sativus L.) was attempted using a foliar spray of uniconazole, a gibberellin biosynthesis inhibitor. At first, appropriate conditions for the prevention of precocious bolting, such as uniconazole concentration, extent of vernalization and floral stage were investigated using an incubator. Higher uniconazole concentration (≧17 µM) and shorter duration of vernalization were found to be desirable. For effective retardation of bolting and flowering, uniconazole should be applied before floral evocation. Based on these results, uniconazole was applied to cv. ‘Taibyo-sobutori’ and cv. ‘Oshin’ during winter and spring cultivation. Bolting and flowering were delayed up to 10 and 6 days, respectively, when the uniconazole (17 µM) treatment was administered before floral evocation.