We tested the hypothesis that supplementation with protein improves fermentation parameters without damaging the rumen microbial populations of beef cattle grazing Urochloa brizantha cv. Marandu during the dry season. Four rumen-cannulated Nellore bulls (571 +/- 31kg of body weight) were used in a 4x4 Latin square design. The treatments were not supplemented with concentrate (only free-choice mineral salt ad libitum) and supplemented (supplements with low-LPSU, medium-MPS, and high protein supplementHPS), supplying 155, 515, and 875g/animal/day of crude protein (CP), respectively. The abundance of each target taxon was calculated as a fraction of the total 16S rRNA gene copies in the samples, using taxon-specific and domain bacteria primers. There was no difference (P>0.05) across treatments for intakes of dry matter (DM), forage and neutral detergent fiber (NDF), digestibility of DM and NDF, and ruminal pH. Animals supplemented with concentrate had greater (P<0.05) intakes and digestibility of CP, ether extract and non-fibrous carbohydrate contents of the substrates (EE+NFC), and ruminal ammonia nitrogen (RAN) compared to control. Bulls that received only mineral salt had lower proportions of Butyrivibrio fibrisolvens and had greater (P<0.05) proportions of Fibrobacter succinogenes, Ruminococcus albus, Ruminococcus flavefaciens, Methanogen archaea than bulls supplemented with concentrate. The MPS animals had greater (P<0.05) intake and digestibility of CP, RAN concentration, and had lower (P<0.05) proportions of Fibrobacter succinogenes, Ruminococcus flavefaciens, and Selenomonas ruminantium than LPSU animals. The HPS provided higher (P<0.05) intake of CP, RAN and proportion of Ruminococcus albus when compared with MPS. In conclusion, supply of 515g/animal/day of protein via supplement provides better ruminal conditions for the growth of cellulolytic bacteria of bulls on pasture during dry season.