Three rumen fistulated Polish Merino wethers were used to study the digestion of hay in situ. The animals were either defaunated or faunated only with Eudiplodinium maggii, with Eudiplodinium maggii and Entodinium caudatum and with both entodiniomorphids plus Dasytricha ruminantium. The diet consisted of hay (750 g) and ground barley (130 g) per meal and was given every 12 h. An in sacco technique was applied to measure the disappearance of dry matter, ADF and NDF from hay samples incubated for 12, 24 and 48 h in the rumen. The bags were either of 42 or 206 μm pore diameter. Numbers of Entodinium caudatum, Eudiplodinium maggii and Dasytricha ruminantium in the rumen contents were 52.4-55.5, 1.4-2.3 and 6.1 x 10⁴/g, respectively. The concentration of Entodinium caudatum and Dasytricha ruminantium inside the bags was not influenced by pore size and varied in the range of 29.4-57.0 and 2.0-5.3 x 10⁴/g, respectively. Number of Eudiplodinium maggii inside the bags of 42 and 206 μm pore size was 0.1-0.4 and 0.6-2.6 x 10⁴/g. Colonization of hay by the fibrolytic bacteria was determined as the activities of particle associated CMC-ase and xylanase. They were the range of 1.6-8.1 and 10.1-51.8 μmol reducing sugars/g DM/min, respectively. Establishment of Eudiplodinium maggii alone or together with Entodinium caudatum increased colonization whereas appearance of Dasytricha ruminantium decreased it to the level observed in the ciliate-free sheep. Disappearance of DM, ADF and NDF from hay incubated in sacco varied in the range of 38.2-75, 18.4-57.6 and 20.8-68.9% in relation to the time of incubation and microfauna composition. Eudiplodinium maggii alone increased significantly the ADF disappearance after 12 and 24 h of incubation while the loss of DM and NDF was either not influenced or reduced. Microfauna composed of Eudiplodinium maggii and Entodinium caudatum supported ADF digestion after 12 h of incubation, however, to a significantly lesser extent than Eudiplodinium maggii alone. Loss of DM and NDF was not influenced or diminished in relation to the time of incubation or porosity of the bags used. Microfauna consisting of the both entodiniomorphids and Dasytricha ruminantium significantly reduced disappearance of all three components independently of the incubation time. Most visible differences were, however, observed after 12 h of incubation. Enlargement in the pore size and thus in the numbers of Eudiplodinium maggii inside the bags did not increase neither DM nor ADF and NDF disappearance from the hay samples incubated in sacco.