It is now accepted that the microbial communities living within the gastro-intestinal tract (GIT) can influence pig health, especially at stressful times such as weaning. Careful design of the animal diet is considered to be an easy way to influence the microflora positively. In this work, fermentable carbohydrates were selected after an in vitro test (cumulative gas production- Bosch et al., 2002) for both rate and extent of fermentability. The ingredients were chosen both for their end-products and in terms of their fementation kinetics so as to stimulate fermentation along the entire GIT. They were incorporated into a test diet (TEST), and compared with a control diet (CONT) which contained minimal fermentable carbohydrates. Digesta were analyzed both in terms of microbial species (molecular techniques) and their end-products (e.g. lactic acid), to determine whether fermentation had, in fact, been stimulated by the TEST diet.