In this study, two experiments were conducted. The first one was performed to isolate pure colonies of Fibrobacter succinogenes S85 from rumen liquor of adult rams and bucks. Samples were collected by stomach tube from 3-3.5 years old rams and bucks at the Agriculture Experimental Station, Faculty of Agriculture, Cairo University, under sterilized conditions. In the second experiment, cellulase gene alignment, PCR primer design and phylogeny tree for cellulolytic ruminal bacteria was performed. PCR and electrophoresis separations were done to assure the target strain to quantify the cellulase gene expression in Fibrobacter succinogenes S85 using Real Time PCR. At the end of the experiment, it was concluded that, the high digestion efficiency in bucks than rams are due to the cellulase gene expression in Fibrobacter succinogenes S85 derived from adult buck's rumen liquor being 3.24- fold higher than the same from adult ram's rumen liquor. Also, the phylogenetic tree clarifies the relationship between cellulolytic bacteria on basis of cellulase gene sequence alignment.