The contents of short-chain fatty acids (SCFA) in the caecal digesta of rats fed diets containing 4 or 10% cellulose (control diets) or 10% apple pulp (P), apple pulp (V), apple pectin, potato fibre or potato pulp were measured. The diets were fed for 27 days to 7 groups of 10 rats each. The animals were sacrificed and the caecum was removed, tissue and digesta weighed, and caecum analysed for SCFA. Caecal tissue and digesta weight, pool of acetic, and propionic acids were highest with pectin- and the lowest with cellulose diets, while potato fibre and apple pulp gave intermediate levels. The proportions of acetic acid were higher and butyric acid lower on pectin than on the other diets. It is concluded that fibre from plant by-products differs in ability to form short-chain fatty acids in the caecum of rats, which is reflected in the amount of fermentable material reaching the caecum.