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Molecular genetic techniques (NGS sequencing and quantitative PCR) were used to determine the composition of the cecal bacterial community of broiler chickens fed with different mixed fodder. The Cecal microbiome exhibited taxonomic diversity, with both typical inhabitants of avian intestine belonging to the families Clostridiaceae, Eubacteriaceae, and Lactobacillaceae and to the phylum Bacteroidetes, and new un- identified taxa, as well as bacteria of the families Lachnospiraceae and Ruminococcaceae, which were previ- ously considered restricted to the rumen microflora. Contrary to traditional concepts, enterococci and bi- fidobacteria were among the minor components of the community, lactate-fermenting species were absent, and typical avian pathogens of the genus Staphylococcus were detected but seldom. Members of the family Suterellaceae and the genus Gallibacterium, which are responsible for avian respiratory infections, were also detected. Significant fluctuations of abundance and composition of microbial groups within the cecal com- munity and of the parameters of broiler productivity were found to occur depending on the feed allowance. Cellulose content in the feed had the most pronounced effect on the composition aid structure of bacterial communities. Decreased cellulose content resulted in a decrease of bacterial abundance by-aii order of mag- nitude and in increased ratios of members of the phylum Bacteroidetes and the family Clostridiaceae, which possess the enzymes degrading starch polysadcharides. Abundance of the normal inhabitants of avian intes- tine belonging to the genus Ldctobacillus and the order Bacillales decreased, while the share of Escherichia and members of the family Sutterellaceae increased, including some species capable of causing dysbiotic changes in avian intestine. No significant change in abundance of cellulolytics of the families Ruminococca- ceae, Lachnospiraceae, and Eubacteriaceae was observed.