1. Four human subjects on strictly controlled diets were given a fibre supplement, 25 g Ispaghula husk (Isogel)/d, for 3 weeks. 2. Replicate diets and faeces were collected during two 5 d balance periods. The first period served as control for the second which occurred after the supplement had been fed for 2 weeks. 3. Diets and faeces were analysed for total solids, gross energy, total nitrogen, fat, available and unavailable carbohydrates. 4. Three of the four subjects showed a considerable increase in faecal bulk; total faecal weight was more than doubled in two subjects. In two subjects the increase was brought about mainly by extra fibre in the faeces accompanied by a higher proportion of faecal water whereas in one subject it was also an increased excretion of nutrients. One subject showed very little change in faecal bulk. 5. High values for the apparent digestibility of fibre were found during the control period; from 0.70 to 0.80. There was more variation during the experimental period when apparent digestibility of total fibre ranged from 0.47 to 0.82. Values arrived at for the minimum apparent digestibility of the fibre supplement were generally high; one subject appeared to digest Isogel completely. Isogel was degraded preferentially to the food-derived fibre; in particular, cellulose in the faeces was increased during the experimental period. 6. Only one subject showed distinct decreases in the apparent digestibility of energy, N and fat in the diet. The results do not therefore agree with the generally held view that increased fibre in the diet decreases the apparent digestibility of the other nutrients.