Divergenl genelic seleclion for wool growth as a single Irail has led lo major changes in sheep physiology, including varialions on rumen microbial prolein produclion and uplake of amino nilrogen in portal blood, This sludy was conducled lo delermine if sheep wilh differenl genelic meril for wool growth under commercial condilions have dislincl rumen baclerial populalions, Eighleen Merino welhers were separaled inlo groups of conlrasling eslimaled breeding values for c1ean fleece weighl (CFW; Low-FW and High-FW) and fed an oaten:lucerne chaff diel al two levels of inlake (U; 1,Q-M or 1,5-M mainlenance) for lwo seven-weeks periods in a crossover design, Baclerial diversily, in fil1ered rumen fluid collecled by esophageal inlubalion, was characlerised using 454 amplicon pyrosequencing of lhe V3N4 regions of lhe 16S rRNA gene. Dala was analysed using Molhur, Acacia, Qiime and SAS software packages. Baclerial diversily eslimaled by Phylogenelic dislance, Cha01 and Observed species did nol differ significantly wilh CFW or U; however, the Shannon diversily index differed (P=0.0432) between High- (7.67) and Low-FW sheep (8.02). High-FW animals had a higher (P=0.0342) proportion of Bacleroideles (71.9 vs 66.5%) and a lower (P=0.0359) proportion of Firmicules (26.6 vs 31.6%) lhan Low-FW animals. Twenly-four specific OTUs, belonging lo Firmicules and Bacleroideles phyla, were shared among alllhe samples, whereas lhirty-seven specific OTUs varied significanlly in presence/abundance (P