Effects of commercial antimicrobials and the individual and combinational use of commercial prebiotics and probiotics in feed from d zero to 41 on the growth performance, small intestine size, jejunal morphology, and ileal resident bacteria population of broiler chickens were determined. A total of 1,040 one-day-old male Ross × Ross 708 broilers were randomly distributed to 80 floor pens (5 treatments, 16 replications per treatment, 13 chicks per pen). Five dietary treatments were employed: 1) a corn soybean-meal basal diet (served as a negative control diet, NC); 2) a basal diet supplemented with a commercial prebiotic product (Pre); 3) a basal diet supplemented with a probiotic product containing Bacillus subtilis spores (Pro); 4) a basal diet supplemented with both prebiotic and probiotic products (Pre + Pro); and 5) a basal diet supplemented with commercial antimicrobials (served as a positive control diet, PC). At d 14, Pre diets improved the relative level of Lactobacillus in ileal mucosa as compared to NC, Pro, or PC diets (P = 0.045) without improving broiler BW. Broilers fed PC diets exhibited the highest BW gain from d 15 to 27, the lowest duodenum, jejunum, and ileum relative weights as percentage of BW at d 27, and the highest breast weight at d 42 (P = 0.026, 0.035, 0.002, 0.025, and 0.035, respectively). Broilers fed Pro or Pre + Pro diets exhibited higher BW gain from d 28 to 41 (P = 0.005) and higher overall BW gain from d zero to 41 (P = 0.039) than those fed other diets. Dietary treatments did not affect jejunal morphology or ileal resident Escherichia coli level at any age. From our results, including spores of Bacillus subtilis in feed may stimulate growth at a later age and may facilitate broilers in reaching their target weight sooner. Therefore, probiotics are recommended as potential alternatives to antimicrobials in chicken diets, especially in grower and finisher feed.