A feeding trial was performed on 60, Cobb 500 broiler chicks (14-28 days) assigned to 2 groups (C, E) housed in an experimental hall with 32˚C air temperature, 36% humidity and 23 h light regimen. The conventional diet (C), with corn and soybean meal as basic ingredients, had 3082.48 kcal/kg metabolisable energy and 19.99% crude protein. Unlike the diet of C group, the diet of experimental group (E) had 1% willow bark extract (Salix alba). At the age of 28 days, 5 broilers/group were slaughtered and samples of caecal content were collected for bacteriological examination. Compared to group C, the pathogenic bacteria, Enterobacteriacee and Escherichia coli (colony forming units), were significantly (P≤0.05) lower in the caecum content, while the units of lactobacilli were significantly (P≤0.05) higher in group E. Throughout the experimental period, under heat stress, no mortalities were recorded.The inclusion of 1% willow bark extract in broiler diets (14-28 days) reared under heat stress reduced the proliferation of pathogenic bacteria and stimulated the growth of favourable bacteria such as lactobacilli in the gut.