The physical disintegration of five mature, chaffed roughages (barley straw, oat straw, pea straw, ryegrass hay and lucerne hay) in chewing during eating was examined using four oesophageal-fistulated steers. Measurements were made of intake, chemical composition, modulus of fineness and grinding energy of the feeds, and of particle size distribution, modulus of fineness and the proportion of particles greater than 1 mm in the oesophageal extrusa. Significant differences between steers and between feeds occurred in modulus of fineness and the proportion of particles greater than 1 mm. Between steers, there were decreases, with chewing, of 30-40% in the mean proportion of particles greater than 1 mm. The order of proportionate particle size reduction for the different roughages was as follows: pea straw <: oat straw < ryegrass hay < lucerne hay < barley straw. Expressed in terms of the modulus of fineness, the mean decreases in particle size ranged from 46 to 52% for the different roughages. Some interactions occurred between the different roughages and individual steers. The critical characteristics responsible for the variation that occurred have not been defined either in the roughages or in the steers.