The effects of 100 g/kg diet substitution of date-waste dietary fibre, DW1, untreated (diet II) or after growing Endomycopsis fibuligera on it for 60 hours, DW2 (diet III), on the body weight gain, food efficiency ratio (FER); protein efficiency ratio (PER), food intake; food conversion efficiency (FCE) and apparent digestibility (Dapp) of rats fed a high-carbohydrate diet (I) for 8 weeks were studied. Rats fed on the three diets exhibited good nutritional values during the first 4 weeks. Then these values began to decrease with increasing time of feeding in the cases of diets II and III, only. The results also showed a highly significant decrease in the levels of glucose in sera, and glycogen in the liver and muscles of rats fed diets II and III. The fibres caused a significant decrease in lipid and protein contents in the muscles of rats fed diets II and III over the control diet I. Copyright (C) 1996 Elsevier Science Ltd.