The effects of cellulose added in three levels (7, 15 and 20% crude fibre in dry matter) to three different basal diets (a high fat diet, a high starch diet with raw starch and a high starch diet with cooked starch) on apparent digestibility were investigated in eight adult dogs. Cellulose had little effect on the apparent digestibility of fat. In the high fat diet there was no significant decrease, not even at the highest cellulose level (98.3% compared with 98.1% in the basal high fat diet). In the cooked starch diets, fat digestibility decreased from 95.1% in the basal cooked starch diet to 93.8% at the highest cellulose level. In the raw starch diets, digestibility did not decrease with increasing cellulose levels. The apparent digestibility of crude protein was considerably decreased by cellulose in all diets. Starch also decreased protein digestibility and the effects of cellulose and starch appeared to be additive (high fat diet decrease of protein digestibility from 86.7 to 83.5%, cooked starch from 81.6 to 78.6%, raw starch from 79.0 to 70.8%, basal diets to highest cellulose levels, respectively). The apparent digestibility of nitrogen-free extract decreased from 93.9% in the basal cooked starch diet to 84.5% at the highest cellulose level. The figures for the raw starch diets were similar (decrease from 93.4 to 85.9%). Cellulose decreased the apparent digestibility of energy in all diets. This decrease was more marked in the high starch diets (cooked starch decrease from 89.1 to 69.6%, raw starch from 88.9 to 70.2%) than in the high fat diet (decrease from 90.1 to 76.1%). An evaluation of previous data showed that in general fibre has a higher impact on the apparent digestibility of energy in high carbohydrate diets than in low carbohydrate diets. The apparent digestibility of potassium, sodium and chloride was impaired by cellulose. The apparent digestibility of these minerals tended to be lower in the high starch diets, especially in the raw starch diet, and the effect of cellulose was usually more marked in those diets.