Marine and freshwater fish harbour an intestinal microbiota less abundant than in mammals, made of aerobic and facultative anaerobic bacteria. Limited and conflicting data have shown either the complete lack of cellulose degradation in the trout or tilapia, or a limited (13%) activity in the trout. A digestibility study carried out in the trout and the carp administered a purified (devoid of lignin and reduced amount of hemicelluloses) crystalline cellulose extracted from wood (fibre length ˂ 150 μm, diameter ˂ 45 μm), showed the practical absence of cellulose degradation in both species (Bergot and Breque, 1983).