Aims: Determining the effects of zinc bacitracin, bird age and access to range on bacterial microbiota in the ileum and caeca of broilers. Methods and Results: 16S rRNA gene-based polymerase chain reaction-based denaturing gradient gel electrophoresis (PCR-DGGE) profiling, DNA sequencing and real-time quantitative PCR techniques were used. The richness of both ileal and caecal microbiota increased with chicken age. The microbiota from those birds of the same age demonstrated relatively similar PCR-DGGE profiles and tended to form closely related clusters in the relatedness analyses. Dietary treatment with bacitracin (50 mg kg(-1)) and access to range did not change the richness but altered the composition of the microbiota. The impact of bacitracin was particularly obvious in 3-day-old chicks. Lactobacilli were abundant in the caecal microbiota of 3-day-old chicks regardless of the dietary treatment with bacitracin. The access to range enriched Bifidobacterium in both the ileum and caeca. Conclusions: Bacitracin, bird age and access to range all influenced bacterial microbiota in the ileum and caeca of broilers, with bird age having the greatest apparent effect. Significance and Impact of the Study: Providing useful information for the development of antibiotic replacement therapy for poultry production.