Meale, S. J., Ding, S., He, M. L., Dugan, M. E. R., Ribeiro Jr. G. O., Alazzeh, A. Y., Holo, H., Harstad, O. M., McAllister, T. A. and Chaves, A. V. 2014. Effect of Propionibacterium freudenreichii on ruminal fermentation patterns, methane production and lipid biohydrogenation of beef finishing diets containing flaxseed oil in a rumen simulation technique. Can. J. Anim. Sci. 94: 685-695. The objectives of this study were to examine the effects of Propionibacterium freudenreichii (strain T54; PB) and flaxseed oil (FO) in a total mixed ration on ruminal fermentation, CH₄ production and fatty acid biohydrogenation in two artificial rumens (RUSITEC). The experiment consisted of 8 d of adaptation and 12 d of sample collection with four replicate fermenters per treatment. Treatments were: (1) CON; (2) PB; (3) FO (60 g kg^-1 DM with autoclaved PB); (4) FOPB (60 g kg^-1 DM with PB). Disappearance of DM (g kg^-1 DM) and gas production (mL g^-1 DM) were not affected by treatment (P>0.05). Inclusion of FOPB increased (P=0.01) total volatile fatty acid (VFA) production (mmol d^-1), compared with CON and PB. The acetate:propionate ratio was reduced (P<0.001) in all treatments, compared with CON. Methane production (mL g^-1 DM or mL g^-1 DMD) was lowest (P<0.001) with PB (27.1%); however, FO (14.3%) and FOPB (19.3%) also reduced CH₄ compared with CON. Fatty acid profiles for PB were similar (P>0.05) to CON for most fatty acids. Concentrations of 18:3n-3 were greater (P<0.001) in FO and FOPB in both digesta and effluent, compared with CON. Propionibacterium freudenreichii had very little effect on ruminal biohydrogenation, but reduced CH₄ production under the current conditions as a result of increasing propionate production.