The accumulation of fructo-oligosaccharides and the activities of fructosyltransferase (sucrose[ratio ]sucrose 1F-fructosyltransferase (SST), 1F-fructosyltransferase (1F-FT) and 6G-fructosyltransferase (6G-FT)) in the bulbs of three onion cultivars were investigated from June to September 1993. The total fructo-oligosaccharide content increased from June until August, then decreased in September, except in one cultivar. The levels of neokestose and its related tetrasaccharides (1F, 6G-di-β-D-fructofuranosyl sucrose and 6G (1-β-D-fructofuranosyl)2sucrose) were higher than those of 1-kestose and nystose throughout growth. The activities of 6G-FT, 1F -FT and SST were high in June and July, then decreased; SST activity was very low in September. The activity ratios of 6G-FT to 1F-FT varied between 1·86 and 2·65 over the growth period. Two trisaccharides, three tetrasaccharides and four pentasaccharides were identified, together with a mixture of hexa- and heptasaccharides, all of which were synthesized in vitro from 0·1 M sucrose by an enzyme preparation of onion bulbs harvested in August. Octa- and nonasaccharides other than the saccharides formed from sucrose were also synthesized from 0·1 M 1-kestose or 0·1 M neokestose. All the saccharides produced from sucrose, 1-kestose or neokestose by the crude enzyme prepared from onion bulbs were identical to the saccharides occurring naturally in onion bulbs.