Phenolic contents in developing and mature fruits of peach (Prunus persica Batsch var. vulgaris Maxim.) and nectarine (P. persica Batsch var. nucipersica Schneid.) cultivars were compared in relation to astringency. Total phenolic contents varied widely among the peach cultivars, being low in 'Wase-momoyama, ' 'Setouchi-hakuto, ' 'Takei-hakuho, ' 'Saotome' and 'Shimizu-hakuto' and high in 'Golden Peach, ' 'Hakuto, ' 'Mishima-hakuto, ' 'Koho' and 'Koyo-hakuto' ; 'Kodaira-wase' and 'Okubo' were intermediate. The content of higher molecular fraction of phenolic compounds was positively correlated with the total phenolic concentration. Two nectarine cultivars, 'Shuho' and 'Fantasia, ' had consistently higher levels of total and higher molecular phenolic compounds. The total phenolic contents in 'Shimizu-hakuto' and 'Hakuto' peach fruit decreased toward the end of Stage 1 of their growth curve ; thereafter, they increased rapidly and reached a maximum in the middle of Stage 2, and then rapidly decreased to a lower level during Stage 3. Total and higher molecular phenolic contents were greater in 'Hakuto' than in 'Shimizu-hakuto' throughout fruit growth, especially during Stage 2. Based on these results, the degree of astringency among the peach and nectarine cultivars is discussed.