The phasing out of antibiotic compounds as growth promoters from the animal industry means that alternative practices will need to be investigated and the promising ones implemented in the very near future. Fermentation in the gastrointestinal tract (GIT) is being recognized as having important implications for health of the gut and thus of the host animal. Fermentation in single-stomached animals occurs to the largest extent in the large intestine, mainly because of the longer transit time there. The present review examines the micro-ecology of the GIT, with most emphasis on the large intestine as the most important site of fermentative activity, and an attempt is made to clarify the importance of the microfloral activity (i.e. fermentation) in relation to the health of the host. The differences between carbohydrate and protein fermentation are described, particularly in relation to their endproducts. The roles of volatile fatty acids (VFA) and NH 3 in terms of their relationship to gut health are then examined. The large intestine has an important function in relation to the development of diarrhoea, particularly in terms of VFA production by fermentation and its role in water absorption. Suggestions are made as to feeds and additives (particularly those which are carbohydratebased) which could be, or are, added to diets and which could steer the natural microbial population of the GIT. Various methods are described which are used to investigate changes in microbial populations and reasons are given for the importance of measuring the kinetics of fermentation activity as an indicator of microbial activity.