The in vitro degradation of dietary fibre from three brown seaweeds (Himanthalia elongata, Laminaria digitata and Undaria pinnatiJda) was studied, using human faecal flora. Two sets of fibre were tested:(1) total algal fibres extracted from the whole algae, mainly composed of alginates, and (2) purified fibres (sulphated fucans, Na-alginates and laminarans) representative of those contained in the whole brown algae. Mannuronate, one algal component, was also investigated. Substrate disappearance and shortchain fatty acid (SCFA) production were monitored after 6, 12 and 24 h fermentation. Gas production was followed hourly during the first 9 h and then at 12 and 24 h. Sugarbeet fibre was used as a fermentation reference substrate. According to the fermentative indices used, most of each of the total algal fibres disappeared after 24 h (range 60-76 %) hut, unlike the reference substrate, they were not completely metabolized to SCFA (range 47-62 %). Among the purified algal fibres, disappearance of laminarans was approximately 90% and metabolism to SCFA was approximately 85% in close agreement with the fermentation pattern of reference fibres. Sulphated fucans were not degraded. Naalginates exhibited a fermentation pattern quite similar to those of the whole algal fibres with a more pronounced discrepancy between disappearance and production of SCFA: disappearance was approximately 83 % but metabolism was only approximately 57 YO. Mannuronate was slowly fermented hut its metabolism corresponded to its disappearance from the fermentative medium. Thus, the characteristic fermentation pattern of the total fibres from the three brown algae investigated was attributed to the peculiar fermentation of alginates, and mannuronate was shown not to be directly involved.In vitro fermentation: Faecal bacteria: Dietary fibre: Brown algae Most algal polysaccharides are resistant to hydrolysis by human endogenous digestive enzymes and thus are considered as dietary fibre (Trowel1 et af. 1976; Southgate, 1977; Salyers et al. 1977~-c). The recent approval of ten seaweeds for human consumption in France (Fleurence, 1991) led to a revival of interest in these sources of fibre, and information concerning methods of quantification, amounts and physico-chemical properties of algal dietary fibres has been published recently (Kishi et af. These reports demonstrate that seaweeds are rich in dietary fibre, with a particularly high amount of the soluble forms in brown algae. In these seaweeds, soluble fibre consists of laminarans (Pl-3, Pl-6 glucans associated with mannitol residues; * For reprints.