The yacon plant (Smallanthus sonchifolius) contains inulin andfructooligosaccharides, which are known prebiotics. This study wasdone with the intent of evaluating the prebiotic activity of dried young and mature yacon root ethanolic extracts. The prebiotic activity was evaluated by the effect of plant extracts on the growth of Lactobacillus acidophilus. The prebiotic activity was significantly higher for ethanolic young root extracts at such concentrations compared to that of the ethanolic mature root extracts. At 10% concentration, the young Yacon ethanolic root extract has significantly higher prebiotic activity compared to that of the positive control. The prebiotic activity mature Yacon in all the experimental concentrations used (0.5%, 1%, 2%, 5%, and 10%) were significantly different, all have significantly lower prebiotic means, when compared to that of the positive control. All of the experimental concentrations of young yacon ethanolic root extracts have significantly higher mean of prebiotic activity compared to the negative control. Another purpose of this study was to isolate and characterize inulin from young and mature yacon roots. Isolation was done by aqueous extraction and crystallization. Greater amount of inulin was isolated from young yacon root, which was found to have less impurity in a color basis than that obtained from the mature yacon root. Both the inulin from the young and mature yacon roots has no significant % glucose mean difference compared to commercial inulin.