The exploration of the bovine epimural bacterial microbiota (BEBM) in the rumen is relevant for basic and applied research for explaining the effect of subacute ruminal acidosis (SARA) on BEBM community structure and host-microbe interactions. Comparatively little is known about the metabolic function of the BEBM and whether or how it might be affected by a diet-induced SARA challenge. This study aimed to investigate differences in the gene expression of the BEBM during a diet-induced SARA challenge. Three ruminally cannulated Holstein dairy cows were fed forage (50% grass silage and 50% meadow hay) during a baseline period followed by a 5-week SARA challenge induced by a 60% concentrate diet (barley grain (33.0%), wheat (30.0%), corn (15.0%), and rapeseed meal (17.0%)). Cows were adapted to the high-concentrate diet within the first 6 challenge days. Rumen papillae biopsies were taken at the baseline and after the SARA challenge. The metatranscriptome of the BEBM at the baseline and after the SARA challenge was obtained using Illumina HiSeq sequencing. In total, 291 million reads were obtained, of which 35 million remained after quality control and 2.1 million reads were bacterial reads and were assigned to 25 phyla, with Proteobacteria, Firmicutes and Bacteroidetes being the most abundant phyla. Functional assignments revealed carbohydrate and amino acid metabolism as well as translation to be the most transcribed KEGG/KO pathways. We found that urease was among the highest transcribed genes confirming previous reports of ureolytic activity of rumen wall bacteria. Also genes involved in oxygen scavenging such as superoxide dismutase were among the highest expressed genes. The high abundance of transcripts involved in oxygen scavenging provides evidence that the rumen wall is not a strictly anoxic ecosystem. PCoA revealed clustering of BEBM community structure, which was not reflected in the functional data. A diet-induced SARA challenge largely affects the composition of the BEBM without strong effects on the functional metatranscriptome, indicating that shifts in bacterial community composition are compensated on a functional level.