The aim of this study was to evaluate the effect of live yeast (Saccharomyces cerevisiae CNCM I-1077) supplementation on fiber digestibility and rumen population of cellulolytic bacteria of grazing Nellore cattle, throughout the 4 seasons of the year. Eight rumen cannulated steers were used in a completely randomized design in a 2 × 4 factorial arrangement of treatments: with or without yeast and seasons of the year (spring, summer, fall, and winter). The live yeast product (Levucell SC Farm, Lallemand, Brazil) was given daily, in capsules, via the rumen cannula, to achieve 8 × 10⁹ cfu per animal. Animals were kept on an intensive rotational grazing system throughout the year, receiving similar mineral supplementation. Every 45 d, in situ rumen NDF degradability (NDFD) of 5 reference forages was determined after 24 and 48 h incubation: (1) corn silage, (2) bermudagrass hay, (3) sugarcane silage, (4) marandu-grass, and (5) guinea grass (mombaça-grass). In addition, a composite sample representing the liquid and solid phases of the ruminal content was collected for DNA extraction and real-time PCR quantification of 4 rumen cellulolytic bacteria species (Butyrivibrio fibrisolvens, Fibrobacter succinogenes, Ruminococcus flavefaciens and Ruminococcus albus). On average, live yeast supplementation increased 24 h-NDFD by 6.8% throughout the year (40.7 vs. 38.1%, P < 0.01), without a yeast × season interaction (P = 0.45), or yeast × roughage interaction (P = 0.39). When analyzed after 48h of rumen incubation, there was a yeast × season interaction (P = 0.08). Live yeast supplementation increased 48h-NDFD only during the autumn months (P = 0.01). Among the 4 species evaluated, R. flavefaciens was the most prevalent cellulolytic bacteria, and yeast supplementation increased by 78% (P < 0.01) the relative population of R. flavefaciens in the rumen, and this effect was more pronounced during the summer and spring months (yeast × season interaction, P = 0.10). Supplementation with live Saccharomyces cerevisiae increased fiber digestibility of roughages in steers grazing tropical grasses, and this effect was related to the greater population of R. flavefaciens in the rumen.