1. Groups of chicks were given a low-residue diet with or without supplements of dietary fibre in the form of wheat bran, wheat straw or bagasse. Growth and food conversion efficiency (g weight gained/g food eaten; FCE) during the first 4 weeks of life were measured. 2. In every one of seven experiments supplementation of the diet with 100 g wheat bran/kg resulted in improved growth, and in three experiments FCE was also increased. 3. Supplementation with coarsely-milled wheat straw to provide an amount of unavailable carbohydrate equivalent to that in the bran diet resulted in poorer growth; finely-milled wheat straw had little effect on growth. 4. The growth-promoting effect of bran was destroyed by sterilization with heat or gamma-radiation. 5. In some experiments weights, lengths and volumes of small intestines were measured. Differences in intestinal dimensions between birds given the diet with and without fibre were not consistent, nor were they correlated with growth rate or FCE. 6. Histometric observations on small intestines from a few birds indicated that those given coarse wheat straw had longer vili and thicker muscularis layers, and the caecal tonsils had a greater area of lymphoid tissue and more follicles.