The rumen bacterial composition of both pre-ruminant dairy calves and cows and beef steers was surveyed using pyrosequencing of the 16S rRNA gene. Sequences were analyzed using taxonomy-dependent and-independent clustering methods. The core rumen microbiome, regardless of the rumen developmental status or breeds, consisted of 8 phyla, 11 classes, 15 families, and 17 genera. Principal component analysis and clustering demonstrated that the bacterial communities in the rumen of pre-ruminant dairy calves, dairy cows, and beef steers were clearly distinguishable. Approximately 66% of phyla and 41% of Operational Taxonomic Units (OTUs) in a typical rumen bacterial community differed in relative abundance between the developing and mature rumen. Greater abundance of Fibrobacteraceae and Ruminococaceae in the rumen of beef steers likely reflected the need for enhanced fiber-digesting capacity in beef cattle. Our results should facilitate understanding of the structural and functional relationships in the rumen microbial ecosystem.