P>To obtain basic information about flower colour sports in azalea breeding, the petals of Rhododendron x pulchrum'Oomurasaki' and its red flower sport were compared in terms of pigment constitution, expression of eight genes related to flavonoid synthesis and sequences of the flavonoid 3',5'-hydroxylase (F3'5'H) gene, which is a key gene that synthesizes the pigments of the delphinidin series. According to the HPLC analysis, 'Oomurasaki' contained anthocyanidins from both the cyanidin and delphinidin series, whereas the red flower sport contained only pigments from the cyanidin series. Real-time PCR analysis showed that F3'5'H transcript levels in the red flower sport were 0.14-fold those of 'Oomurasaki'. Compared with the amino acid sequences of F3'5'H1-A of 'Oomurasaki', both F3'5'H1-B and F3'5'H1-C sequences of the red flower sport were changed by only one amino acid. These results suggested that low transcript levels of the F3'5'H gene caused no accumulation of delphinidin series pigments in flower colour change between 'Oomurasaki' and its red flower sport.