SummaryGastrointestinal symptoms and fecal frequency following ingestion of yogurt containing 15g of galacto-oligosaccharides (GOS) per d were observed in 12 healthy volunteers. The effect of GOS on intestinal microflora was also studied in six volunteers. Defecation frequency increased during the administration period, but gastrointestinal symptoms, especially flatulence, also increased. The level of fecal bifidobacteria did not increase by the yogurt intake, but a significant increase was observed in the fecal bacteria growing on MRS media. The results indicate that dietary GOS increase gastrointestinal symptoms and fecal frequency in normal adults and have an effect on intestinal microecosystem. Key Words galacto-oligosaccharides, gastrointestinal symptoms, bifido bacteria, defecation frequency Dietary fiber, as it passes to the colon, improves the colonic function (1). Fructo-oligosaccharides (FOS) are recently characterized components of the diet that are now considered in many countries as prebiotics and as a dietary fiber (2). A prebiotic is a "non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon that have the potential to improve host health" (3). The prebiotics that have been identified today and which have served to introduce this concept are carbohydrates that resist autoenzymic digestion and that are quantita tively fermented in the colon. FOS have been studied intensively over the past decade, and many positive health effects of FOS have been found (4). There is evidence that FOS relieve constipation, reduce the production of toxic substances