The microbiota of the intestinal tract of chickens plays an important role in inhibiting the establishment of intestinal pathogens. Earlier culturing and microscopic examinations indicated that only a fraction of the bacteria in the cecum of chickens could be grown in the laboratory. Therefore, a survey of cecal bacteria was done by retrieval of 16S rRNA gene sequences from DNA isolated from the cecal content and the cecal mucosa. The ribosomal gene sequences were amplified with universal primers and cloned or subjected to temporal temperature gradient gel electrophoresis (TTGE). Partial 16S rRNA gene sequences were determined from the clones and from the major bands in TTGE gels. A total of 1,656 partial 16S rRNA gene sequences were obtained and compared to sequences in the GenBank. The comparison indicated that 243 different sequences were present in the samples. Overall, sequences representing 50 phylogenetic groups or subgroups of bacteria were found, but approximately 89% of the sequences represented just four phylogenetic groups (Clostridium leptum, Sporomusa sp., Clostridium coccoides, and enterics). Sequences of members of the Bacteroides group, the Bifidobacterium infantis subgroup, and of Pseudomonas sp. each accounted for less than 2% of the total. Sequences related to those from the Escherichia sp. subgroup and from Lactobacillus, Pseudomonas, and Bifidobacterium spp. were generally between 98 and 100% identical to sequences already deposited in the GenBank. Sequences most closely related to those of the other bacteria were generally 97% or less identical to those in the databases and therefore might be from currently unknown species. TTGE and random cloning indicated that certain phylogenetic subgroups were common to all birds analyzed, but sequence data from random cloning also provided evidence for qualitative and quantitative differences among the cecal microbiota of individual birds reared under very similar conditions.