This study investigated the effects of normal and slow poultry development on the structural diversity of cecal bacterial flora using different chicken breeds and feeding stages. Cecal samples were collected from normal and slow developing, 4, 6 10, 16, 20, and 40-week-old commercial layers, and 1, 2, 4, 6, 7, and 8-week-old broilers chickens. Polymerase chain reaction and denaturing gradient gel electrophoresis (PCR-DGGE) of bacterial 16S ribosomal deoxyribonucleic acid (rDNA) was used, in combination with cloning and sequencing of amplified fragments to produce bacterial flora DNA fingerprints. Cluster analysis was further used to identify the specific and common community members. Lactobacillus species were more similar in normal chickens of both breeds than in slow developing chickens. A significant difference (P<0.05) in the average 16S rDNA PCR-DGGE fingerprint band number was observed between normal and slow developed chickens of the same breed at different ages. Bacteroides fingerprints indicated similar cecal bacteria in the two breeds of normal and slow developing chickens. Average band numbers for the 16S rDNA PCR-DGGE fingerprints of the two breeds at different ages were not significantly different (P>0.05), while band numbers between normal and slow developed chickens was significant (P<0.05). Clostridium specific fingerprints were not significantly different between 20-week and 40-week-old commercial layers (P>0.05), and significantly different between healthy and poor broiler chickens of different ages (P<0.05). Lactobacillus agilis was most abundant in both normal and slow developing commercial layers during the laying period, while Lactobacillus aviaries and an unculturable bacteria were abundant during the brooding and rearing periods. Lactobacillus animalis, Bacteroides acidifaciens and an unculturable Bacteroides sp., along with unculturable Clostridium were common in both chicken breeds undergoing normal and slow development. Unculturable proteobacteria were also abundant in commercial layers. Shigella sonnei was prevalent in normal broiler chickens, but absent in the slow developing chickens. Substantial differences in the cecal bacterial community composition exist between different chicken breeds at different developmental stages. Furthermore, bacterial population structure potentially has a major influence on chicken growth.