Treatment of straw with alkali; sodium hydroxide or ammonia, can improve both its dry matter digestibility and the amount of straw that cattle will consume. Such improvements allow straw to be used to a greater extent compared with untreated straw in rations for cattle. Several recent reports have identified however that when straw is incorporated in a ration with rapidly fermentable sources of energy, conditions in the rumen unfavourable for the bacterial degradation of straw are generated by the rapidly fermentable material, particularly at high levels of feeding. Such conditions result in inefficient utilization of the straw. It was suggested however (Williams 1984) that low level supplementation using starch, of diets high in fibre would result in an increase in intake of the roughage but that the digestibility of the material would be unaffected. The trials reported here were designed to determine whether low level supplementation of a diet of straw with either fishmeal or starch affected the degradability of fibre or the rate at which undegraded material passed out of the rumen.