Improving the feed efficiency of cattle contributes to the environmental and economic challenges facing ruminant production. Within this context, a study with 99 fattening Charolais bulls was conducted over two consecutive years (n=49 the first year and n=50 the second). Each year, bulls were allocated to two groups fed ad libitum, corn silage, or a grass silage diet. Bulls were tested for residual feed intake (RFI) during 84 days, and extreme RFI animals were identified thereafter (n = 32; balanced for RFI group, diet and year). Total rumen contents from the eight most extreme RFI+ and RFI- bulls on each diet were sampled at the abattoir (4 bulls/diet/year, n=32). All 49 animals from the first year were also sampled for rumen fluid by stomach-tubing. Total genomic DNA was extracted and subjected to PCR amplification using universal primers targeting the 16S rRNA gene prior to Ilumina sequencing. Raw reads were analyzed using the QIIME2 pipeline with default parameters, and genus-level tables for rumen fluid and total contents were separated and used to assess alpha- and beta-diversity in R using vegan package. Effects of diet, RFI group and the interaction diet×RFI were analyzed using the Kruskal-Walis test. For rumen fluid, Simpson and Shannon diversity indexes were higher in corn-fed bulls, but this effect was not observed for total contents sampled at the abattoir. For both groups of samples, permanova analysis highlighted the effect of diet and diet×RFI on rumen microbial community structure, while the effect of RFI was non-significant. Permanova analysis within each dietary group did not show major changes in microbial community structure. Research from this study indicates that there are no shifts in the overall prokaryotic community structure related to RFI. Further analysis will explore the impact of individual microbial taxa on animal’s feed efficiency.