Narvaez, N., Alazzeh, A. Y., Wang, Y. and McAllister, T. A. 2014. Effect of Propionibacterium acidipropionici P169 on growth performance and rumen metabolism of beef cattle fed a corn- and corn dried distillers' grains with solubles-based finishing diet. Can. J. Anim. Sci. 94: 363-369. A growth and metabolism experiment was conducted to evaluate the effect of Propionibacterium acidipropionici P169 on feedlot steers fed a corn- and corn dried distillers' grains with soluble (DDGS)-based finishing diet. Steers (40 non-cannulated and 8 ruminally cannulated) were divided into two groups and administered 10 g head^-1 d^-1 of maltodextrin containing 1×10¹¹ colony-forming units (CFU) of P169 or the same amount of carrier (Control), top-dressed once daily upon feeding. Feed intake, growth rate and feed efficiency were determined over 115 d. For cannulated steers, ruminal pH was monitored continuously for 5 d during the second week of each month over the entire feeding period with rumen samples collected 3 h after feeding on days 1 and 5. Molar proportions of butyrate, branched-chain volatile fatty acids (VFA) and NH₃-N concentration increased (P<0.01) with P169, whereas total VFA, molar proportions of propionate, the acetate:propionate ratio, and lactate concentration did not differ (P>0.05) between treatments. P169 had a limited effect on ruminal pH as duration and area under the curve both at pH 5.5 and 5.2 as well as frequency of acute ruminal acidosis bouts were similar (P>0.05) for both groups. Compared with control steers, steers fed P169 had more (P<0.05) bouts of subacute ruminal acidosis (SARA). All steers had similar (P>0.05) feed intake, growth rate, feed efficiency and carcass characteristics, except for longissiumus muscle area, being less (P<0.05) for P169 steers than controls. Supplementing P169 to beef cattle fed a corn- and corn DDGS-based finishing diet had no effect on growth performance of beef cattle or ruminal pH and increased the bouts of SARA.