The objectives of this study were to evaluate the effect of oak fruit (Quercus persica) in thediet of sheep with and without polyethylene glycol (PEG) on nutrients digestibility, rumenfermentation parameters, protozoal count and microbial protein supply. In Exp. 1, five fistu-lated mature male sheep were used in a 5 × 5 Latin square design, with 5 dietary treatmentsarranged in a 2 × 2 + 1 factorial: (1) control as a basal diet; (2) control plus 10% oak acorn(OA); (3) control plus 30% OA; (4) control plus 10% OA with 100 g PEG; and (5) control plus30% OA with 100 g PEG. Incorporation of OA decreased digestible organic matter (DOM),digestible crude protein (DCP) and digestible neutral detergent fiber (DNDF) and the lowestvalues were observed at the level of 30% OA. However, administration of PEG to the dietsovercame deleterious effect of acorn on these parameters, significantly (P < 0.01). With theaddition of OA a noticeable decrease in ammonia and TVFA concentration was observed andadding PEG to the diets increased these values even higher than those of control (P < 0.01).Incorporation of acorn to the diets lowered urinary purine derivatives (PD) and urinary,retained and absorbed nitrogen and increased fecal and endogenous nitrogen (P < 0.01).Meanwhile, addition of PEG increased PD and urinary, retained and absorbed nitrogen anddecreased fecal and endogenous nitrogen. Total protozoa content was affected in some-what by OA and PEG supplementation. Isotricha, dasytricha and, diplodinium contents didnot differ due to inclusion of OA, however, the trend for entodinium and ophryoscolex wasto be decreased (P < 0.05). There was a significant effect on individual genera of protozoafollowing addition of PEG to the diets. Cellulolytic and proteolytic bacteria were not influ-enced by incorporation of OA but the trend was in decreasing manor. In Exp. 2, in vitrogas production was conducted to evaluate kinetics of gas produced by diets. The parameter“A” did not differ among exper
