The aim of this study was to examine if synbiotics can function as alternatives to antibiotics in broiler production under heat stress (HS). Day-old broiler chicks (528 birds) were randomly placed in floor pens within 2 identical temperature-controlled rooms (11 birds/pen and 24 pens/room). The pens of each room were evenly divided among 3 treatments (n = 8): basal diet (CON), the basal diet mixed with 50 ppm of bacitracin methylene disalicylate (BMD) or a synbiotic (50 ppm of PoultryStar me^US, SYN). From d 15, room 2 was under thermoneutral (TN) conditions (TN-CON, TN-BMD, and TN-SYN), while HS was applied to room 1 at 32^oC for 9 hrs/d (0800 to 1700) (HS-CON, HS-BMD, and HS-SYN). Treatment effects on footpad dermatitis and gait score were measured on 5 birds/pen, and latency to lie (LTL) test was measured on 2 birds/pen at d 27 and d 41; and 1 broiler/pen was sampled on d 28 and d 42 …