Supplementation of the diet with low concentrations of antibiotics stimulates the growth of poultry by affecting the intestinal flora. The bacterial flora of the small intestine of turkey poults was extensively analyzed in an attempt to correlate changes in populations with growth response. Lactobacillus species comprised almost 100% of the duodenal flora of two-week-old poults but there was no difference in species associated with antibiotic (zinc bacitracin, 55 ppm) treatment. The ileal flora also was predominantly lactobacilli (average 75% of the flora). The most common lactobacilli from the turkey intestinal tract were several previously undescribed Lactobacillus species followed by L. acidophilus, L. salivarius subsp. salivarius, L. fermentum, and L. plantarum. Antibiotic treatment resulted in a shift in the proportions of several of the unnamed Lactobacillus sp. Preliminary feed trials using two strains of lactobacilli that belonged to species that increased in numbers with antibiotic treatment did not stimulate growth when one-day-old birds were colonized with the strains. A probable explanation for the increase in growth is the effect of antibiotic treatment on the multiplication of bacteria in the small intestine. As the digesta move from the gizzard to lower ileum an average 16-fold increase in bacteria occurs in untreated birds. In antibiotic-treated birds the increase was only 2-fold. This inhibition of growth is not due strictly to cell lysis because there are no significant differences in microscopic counts, but the viable counts do decrease. As a corollary there is significantly less lactic acid in the lower ileum of antibiotic-fed birds. Antibiotics did not affect total microscopic or viable counts in the crop or ceca. The above experiments were all done with zinc bacitracin; however, the inhibition of bacterial multiplication was also observed with procaine penicillin. The conclusion from my data is that zinc bacitracin, and probably procaine penicillin, stimulate the growth of turkey poults by a general suppression of the small intestinal flora rather than by an effect on any individual bacterial species. / Ph. D.