The digestibility of ispaghula, a mucilage from Plantago ovata composed mainly of arabinoxylans, and its faecal bulking effect were studied in seven healthy volunteers who ingested a low fibre controlled diet plus either placebo or 18 g/day of ispaghula for two 15 day periods. Whole gut transit time and gas excretion in breath and flatus were not different during the periods of ispaghula and placebo ingestion. Faecal wet and dry weights rose significantly, however, during ispaghula ingestion. Faecal short chain fatty acid concentrations and the molar proportions of propionic and acetic acids also increased. Most of the ispaghula had reached the caecum four hours after ingestion in an intact highly polymerised form. During ispaghula ingestion, the increase in the faecal output of neutral sugars was accounted for by the faecal excretion of arabinose and xylose in an intact highly polymerised form; the apparent digestibilities of these sugars were 24 (11) and 53% (6) respectively (mean (SEM)). In conclusion, ispaghula is more resistant to fermentation than previously reported in humans, and its bulking effect largely results from intact material.