The rumen harbors a great diversity of micro-organisms, comprising prokaryotes (bacteria, archaea) and eukaryotes (protozoa, fungi), which cover essential functions for their host. Part of these micro-organisms is specialized in the degradation of plant polysaccharides and thereby constitutes a keystone community for the energy supply to the host. Overall, the genomes of fibrolytic microorganisms harbor hundreds of genes encoding carbohydrate active enzymes (CAZymes), mainly glycoside hydrolases (GH) and carbohydrate esterases (CE) which act synergistically to deconstruct dietary cellulose and hemicelluloses. In young animals, the rumen microbiota establishes from birth. The factors influencing microbial colonization of the rumen are not well known, but it is believed that repeated contacts with the dam or older animals ensure optimal inoculation of functional microbes. Separation of newborns from the dam and distribution of milk replacer could impair rumen microbial colonization in early life, which would affect rumen function and notably fiber degradation process, which would ultimately impact animal health and performance. In this context, feed additives such as probiotics could be used as a strategy to optimize rumen microbial establishment. To this aim, a study was carried out in 16 newborn lambs separated from the mothers before 24h of life and allocated to two groups, control and probiotic. The probiotic supplement was a specific combination of a live yeast additive (S. cerevisiae CNCM I-1077) and yeast metabolites and was distributed through the milk replacer and the starter feed, and lambs were weaned at 42d of age. Rumen samples were collected from birth to 56 days of age in order to analyze microbiota establishment by qPCR and DNA sequencing technology and data presented in the Beneficial Microbes Conference in 2016 have shown that probiotic treatment was beneficial for promoting establishment of fibrolytic bacteria, fungi and protozoa. Samples collected at 56d were also analyzed using an in house-developed DNA microarray targeting CAZyme families known to contain very efficient cellulases and hemicellulases and present in the major rumen fibrolytic microorganisms. The current version of the DNA microarray allows the detection of 394 genes and it targets the coding sequence of catalytic domains from 8 CAZyme families involved in cellulose and hemicellulose degradation (i.e. glycoside hydrolases GH5, GH9, GH10, GH11, GH43 and GH48, and carbohydrate esterases CE1 and CE6). Here we present the DNA hybridization data obtained from 4 lambs of each group, which indicate different GH and CE gene profiles between the two groups, suggesting that lambs supplemented with probiotics harbor a greater fibrolytic potential that is key to the expression of efficient fiber digesting capacities.