Three Polish Merino wethers, fed hay and ground barley, were defaunated and then faunated with Eudiplodinium maggii only; with Eudiplodinium maggii and Entodinium caudatum or with both opryoscolecids and Dasytricha ruminantium. The total number of ciliates varied in the range of 16-403 x 10³/g rumen content depending on the experimental period. Total bacterial contributed to 12.7-19.6% DM of rumen digesta. The bacterial matter in two of three sheep was negatively influenced by Entodinium caudatum and Dasytricha ruminantium while the effect of Eudiplodinium maggii was positive. The establishment of ophryoscolecid ciliates in the rumen resulted in an increase in both CMC-ase and xylanase activities (P<0.01), while the appearance of Dasytricha ruminantium was accompanied by a decrease in xylanolytic activity. A positive relationship was found between the numbers of Eudiplodinium maggii and both CMC-ase and xylanase activities (P<0.05) irrespective of the presence or absence of other protozoa in the rumen. It was calculated that 8-38% of the activity of CMC-ase and 14-49% of xylanase in the rumen originated from Eudiplodinium maggii. The quantities of dry matter, ADF and NDF that disappeared from the rumen within 12 h after feeding were 0.62-0.83, 0.18-0.32 and 0.32-0.54 kg, respectively. Establishment of Eudiplodinium maggii in the rumen was accompanied by increased disappearance of DM, ADF and NDF in only one of three animals. A decrease in disappearance of DM, ADF and NDF was observed following the establishment of Dasytricha ruminantium in all three animals. The concentration of acetic, propionic and butyric acids was within the ranges of 5.25-7.94, 1.29-2.34 and 0.75-1.39 mM/100 ml of rumen fluid, respectively. Acetate and propionate were the lowest in the sheep with the established population of Eudiplodinium maggii, whereas butyrate, in animals with all three ciliate species in the rumen. The highest concentration of acetic acid was observed following establishment of both entodiniomorphid species, while that of butyrate, in sheep inoculated with Eudiplodinium maggii. The production rate of acetate, propionate and butyrate was 21.3-24.5, 4.7-6.6 and 2.8-5.6 μM/g rumen contents/h, respectively. Production of acetic acid was not influenced by protozoa. An increase in the production rate of butyrate at 4 h after feeding was found after establishment of Eudipoldinium maggii in the rumen. Appearance of Entodinium caudatum and Dasytricha ruminantium decreased the production of butyrate at 4 h after and propionate just before feeding, respectively.