This study examined whether different corn silage to grass silage ratios in ruminant rations and different grinding levels of the feed affect the composition of the ruminal Bacteroides-Prevotella community in vitro. Three diets, composed of 10% soybean meal as well as of different corn silage and grass silage proportions, were ground through 1mm or 4mm screened sieves and incubated in a semi-continuous rumen simulation system. On day 14 of the incubation microbes were harvested by centrifugation from the liquid effluent of fermenter vessels. Microbial DNA was extracted for single strand conformation polymorphism (SSCP) analysis of 16S rRNA genes followed by sequencing of single SSCP bands. Fluorescence in situ hybridization (FISH) and real-time quantitative (q) PCR were used to quantify differences in the relative abundance of Bacteroides-Prevotella and Prevotella bryantii. SSCP profiles revealed a significant influence of the forage source as well as of the feed particle size on the community structure of the Bacteroides-Prevotella group. Different, phylogenetically distinct, so far uncultured Prevotella species were detected by sequence analysis of several treatment-dependent occurring SSCP bands indicating different nutritional requirements of these organisms for growth. No quantitative differences in the occurrence of Bacteroides-Prevotella-related species were detected between diets by FISH with probe BAC303. However, real-time qPCR data revealed a higher abundance of P. bryantii with increasing grass silage to corn silage ratio, thus again indicating changes within the community composition of the Bacteroides-Prevotella group. As P. bryantii possesses high proteolytic activity its higher abundance may have been caused by the higher contents of crude protein in the grass silage containing diets. To conclude, results of this study show an influence of the forage source on the ruminal community of Bacteroides-Prevotella. Furthermore, they suggest an effect of the feed particle size on this bacterial group.