Wethers were subjected to experiments aimed at studying the possibility of replacing part of bulk fodder by treated beech sawdust and its effect on ruminal fermentation in the case of administration of chopped and unchopped hay. For a period of 24 weeks the animals consumed 1000 g of diet dry matter daily; the diet contained 41.81% of chopped meadow hay, 25.28% of barley, 15.37% of chopped straw, 14.98% of molasses, 1.32% of urea and 1.24% of mineral supplement. In the test group the 15.37% proportion of straw was replaced by sawdust. In the subsequent ten-week period the animals were given the same diets, but the hay was not chopped. On the whole the experiment was performed with two groups with straw and two with sawdust. In the groups with chopped hay the sawdust in the diet increased the molar percentage of acetic acid, reduced the molar percentage of propionic acid (which resulted in an increased acetate: propionate ratio), and reduced the pH value and energy efficiency of the produced volatile fatty acids (VFA). The levels of total VFA were not affected. In the groups fed unchopped (long) hay the diet with 15.37% of sawdust showed a significant increase in total VFA and in the molar percentage of butyric acid as well as in the energy efficiency of VFA in rumen. The acetate: propionate ratio was not changed, probably owing to sufficient production of acetic acid. Although some variability was recorded in the results, it can be stated that treated beech sawdust had no adverse effect on rumen fermentation and that the replacement of part of bulk forage by this sawdust was successful. A relationship was also found between sawdust and the physical form of the bulk fodder contained in the ration.