The colonization of Lactobacillus spp., enterobacteria and facultatively anaerobic gram-positive cocci was monitored in intestinal samples of growing broiler chicks from 24 h to 28 days of age. Rapid bacterial growth occurred within the first week, followed by stabilization and decline of colony forming units (CFU). Xylanase supplementation led to significantly lower CFU per gram of wet weight for total presumptive enterobacteria and total gram-positive cocci in luminal and tissue samples in the first 3 weeks. Lactobacillus spp. colony counts from tissue samples were higher for animals with the xylanase-supplemented diet, but luminal CFU were not. The composition of dominant Lactobacillus spp. strains was different in duodenal and jejunal tissues, but distribution of Lactobacillus spp. colony forms was unaffected by xylanase treatment. Mucosa-associated Enterococcus spp. displaced the dominant gram-positive cocci in the jejunal samples. d- and l-lactic acid and acetic acid concentrations were significantly higher in ileal samples from the control group on days 7 and 14, while butyric acid concentrations were higher in the xylanase-treated group. It is concluded that the less viscous intestinal environment caused by the xylanase slowed proliferation of gram-positive cocci and presumptive enterobacteria in enzyme-supplemented animals in the first 3 weeks of life.