Technical Abstract: The objective of this study was to evaluate the degradation dynamics of two commonly-fed corn (co)products fermented in vitro and investigate the shifts of the rumen bacterial populations. Previous studies have indicated that processing methods of ruminant feeds directly affect the substrate availability and fermentation rates of corn products and that fat content of ruminant feeds can affect nutrient digestibility, but little information is available regarding the changes in the ruminal microbial population. In this study, intact and defatted forms of two dried distiller grains plus solubles (DDGS) of different processing methods (BPX and HP) were fermented in replicates for 24 or 48 h using the in vitro gas production (IVGP) technique. The IVGP is based on the assumption that the gas produced from the anaerobic fermentation is directly related to the amount of substrate fermented. Fermentation profiles of the DDGS were compared to alfalfa hay. The 16S rDNA bacterial tag-encoded FLX amplicon pyrosequencing technique (bTEFAP) was used to determine the bacterial profile at 24 and 48 h of the in vitro fermentations. Bacterial populations were analyzed at the species level; a bacterium was analyzed only if its DNA comprised more than 5% of the total DNA recovered. Bacteria were grouped by their reported substrate affinities (guilds) for cellulose, hemicellulose, pectin, starch, sugars, protein, lipids, and lactate. Fermentation profiles indicated that removal of lipids before incubation reduced lag time (P = 0.02) and the fractional fermentation rate (P = 0.02) of the low heat-processed feed (BPX). Defatting of DDGS increased the fiber carbohydrate and proteolytic bacteria guild activity (P < 0.01) and decreased lactate-utilizing bacteria populations (P = 0.02). Information regarding fermentation kinetics and bacterial population shifts may result in more precise ration formulations for ruminants using more advanced nutritional models.