This study was conducted to determine whether avilamycin, which is used as a growth promoter in broiler chickens, would affect the structure of the bacterial community within the jejunal intestinal tract. Sixty chickens were assigned to 2 groups: a control group and an avilamycin-treated group. The jejunal tissue from 4 chicks randomly sampled on d 1, 3, 7, 21, and 42 from each treatment group were investigated for changes in villus height, total mesophilic bacteria, lactic acid bacteria, and the structure of the microbial community using PCR-denaturant gradient gel electrophoresis. Supplementation with avilamycin had no effect on villus height and total bacterial count in either the mesophile or lactic acid bacteria. Based on PCR-denaturant gradient gel electrophoresis analysis, 3 bacterial groups were identified in both groups of chicks: gram-positive bacteria having low contents of guanine and cytosine, gamma proteobacteria, and actinobacteria. The abundance of Weisella, Enterococcus faecium, and Pediococcus acidophilus found only in the control group indicated that avilamycin did affect their growth and led to low microbial diversity among the low guanine and cytosine group compared with the control group. In contrast, however, avilamycin treatment was associated with higher microbial diversity among the gamma proteobacteria group in respect of the various Pseudomonas species. This finding should increase awareness of the potential effects of antibiotic use and particularly avilamycin in commercial poultry production in tropical areas. [ABSTRACT FROM AUTHOR]