The objective was to compare effects of 2 Trichoderma reesei enzyme preparations (EFE) on the ruminal planktonic (LIQ) or the weakly (ASO) or tightly (SOL) feed-adhered ruminal bacterial community (fractions; FRC) of lactating dairy cows. The xylanase activities of the moderate (MIX) and high-xylanase (XYL) EFE tested were 11 and 27 mmol of sugar released/min per g, respectively and both improved milk production from a dairy cow diet in a previous study. Three ruminally cannulated dairy cows (159 ± 47 DIM) were assigned to Control (CON), MIX or XYL treatments in an experiment with a 3 × 3 Latin square design with 23-d periods. The MIX and XYL EFE were sprayed on the TMR just before feeding at rates of 3.4 and 1 mL/kg of DM, respectively. Bacterial diversity of rumen samples was determined using the 16S rRNA gene (V1-V3 region) and the Illumina MiSeq sequencing platform. The data were analyzed with a model that included fixed effects of EFE, FRC, their interaction and random effects of cow and period. The XYL increased (P < 0.05) relative abundance (RA, %) of Paraprevotellaceae (5.06 vs. 3.69 ± 0.81), Spirochaetaceae (4.25 vs. 2.46 ± 0.48) and tended (P = 0.11) to increase those of Prevotellaceae (31.8 vs. 24.1 ± 2.82) versus the CON. The MIX decreased (P < 0.05) RA of Ruminococcaceae (4.6 vs. 9.1 ± 2.23) and unidentified Clostridiales (4.48 vs. 6.61 ± 1.43), and tended to increase (P = 0.06) that of Succinivibrionaceae (1.91 vs. 0.55 ± 3.43) versus the CON. The SOL had higher RA (P < 0.05) of Lachnospiraceae (13.1 vs. 5.9 ± 1.86), Veillonellaceae (8.5 vs. 3.2 ± 1.69), unidentified Clostridiales (6.96 vs. 3.53 ± 1.43), and Spirochaetaceae (3.4 vs. 1.5 ± 0.48) and lower RA for unidentified Bacteroidales (3.5 vs. 10.0 ± 1.49) versus LIQ. The XYL and MIX EFE only tended to reduce the RA of Veillonellaceae versus that of CON (6.3 and 5.3 vs. 13.9 ± 2.54) in SOL (FRC × EFE; P = 0.07). The MIX EFE had lower phylogenetic diversity versus XYL and CON (48.6 vs. 51.6 and 51.2 ± 1.47; P = 0.03). The weighted UniFrac distance showed only community structure differences between FRC (P < 0.01). Both XYL and MIX modified the bacterial community profile and diversity of the rumen fractions.