Aims: The effects of partial replacement of a barley-based concentrate with flaxseed-based products on the rumen bacterial population of lactating Holstein dairy cows were evaluated. Methods and Results: Treatments fed were CONT, a normal diet that included barley silage, alfalfa hay and a barley-based concentrate that contained no flaxseed or faba beans; FLAX, inclusion of a nonextruded flaxseed-based product containing 55.0% flaxseed, 37.8% field peas and 6.9% alfalfa; EXT, similar to FLAX, but the product was extruded and EXTT, similar to FLAX, but product was extruded and field peas were replaced by high-tannin faba beans. The rumen bacterial population was evaluated by utilizing 16S rRNA gene sequencing. Most abundant phyla, families and genera were unaffected. However, some taxa were affected; for example, unsaturated fatty acid content was negatively correlated with Clostridiaceae, and tannin content was negatively correlated with BS11 and Paraprevotellaceae. Conclusions: Predominant rumen bacterial taxa were not affected, but the abundance of some taxa found in lower proportions shifted, possibly due to sensitivity to unsaturated fatty acids or tannins. Significance and Impact of the Study: Flaxseed-based products were effective for partially replacing barley-based concentrate in rations of lactating dairy cows. No negative effects of these products were observed on the abundance of predominant rumen bacterial taxa, with only minor shifts in less abundant bacteria.