In order to determine the efficiency of benzoic acid, a feeding experiment was carried out on 400 one-day-old Cobb 500 cockerels. The birds were divided into four dietary treatments: control without additives, or with 2.5, 5 and 7.5 g of benzoic acid per kg diet, respectively. Performance was similar in birds fed the control diet and that with 2.5 g benzoic acid. The dietary inclusion of 5 and 7.5 benzoic acid per kg depressed growth and increased the feed conversion ratio (P<0.05). The pH of the caecal contents tended to decrease following increased benzoic acid supplementation. No differences were found in the pH of ileal and gizzard digesta. Lactic acid bacteria populations in the caecal contents were most numerous in the group fed the diet with 7.5 g benzoic acid (P<0.05). Coliform bacteria tended to decrease in the crop and ileal contents following increased benzoic acid supplementation, while there was no effect on the caecal contents.