Investigations were conducted to gain basic data for improving the berry coloration of seed-less 'Pione' grapes treated with gibberellic acid. Four mature 'Pione' vines each of two strains growing in the same vineyard were selected for this study. the strains bore well- and poor-ly colored berries annually. Shoot growth and berry development on each vine were inves-tigated throughout the growing season.No significant differences in shoot elongation, increment of leaf area per shoot, and changes in chlorophyll content of leaves between the two strains were observed. Yield per ground area was slightly higher in vines of superior berry coloration than in those of inferior ones. 'Veraison' in the latter was five days later than in the former, but no notable difference in berry size between the two strains was observed. Index of skin color based on color chart was higher in vines of superior berry coloration than in those of inferior one throughout the ripening stage; it increased significantly in the former two to three weeks after veraison. At harvest, anthocyanin content of berry skins in vines of superior berry coloration was about twice that of inferior ones. Changes in total soluble solids contents of berries closely accom-panied changes in color and/or anthocyanin level of the skins. Total sugars, glucose, and fruc-tose contents of the pulp were also higher in vines of superior berry coloration than in those of inferior one, but any differences in ratios of sugars between the two strains were not sig-nificant.