The effects of organic acid, probiotic and a combination of the two on performance and gut morphology in broiler chickens were investigated. Two hundred and forty one-day-old Arbor Acre broiler chicks were randomly assigned to five dietary treatments with six replicates, each with eight birds. The treatments were basal diet (negative control, NC), basal diet + antibiotic (positive control (PC)), NC + 0.4% organic acid (OA), NC + 0.3% probiotic (PB) and NC + 0.4% OA and 0.3% PB. Reduced body weight gain (BWG) was recorded for birds on the NC diet at the starter phase and over the total period. The addition of OA significantly increased BWG compared to values obtained in birds on the NC and other diets. Diet had no effect on BWG at the grower phase or on feed intake, dry matter intake and feed conversion ratio in any growth phase. Gain : feed ratio was lowered in the NC diet, but improved significantly by OA and PB in the starter phase. Organic acid supplementation reduced the weight of the bursa of Fabricius. The weight of pancreas, height of villi and crypt depth were reduced in birds on the NC diet compared with those on OA, PB and a combination of these. Diet had no effects on the weights of the lungs, heart, spleen, kidneys, liver, villus width, villus height : crypt depth ratio, lactic acid bacteria, coliform bacteria and total bacterial count in any gut section. In conclusion, supplementation of broiler diets with OA and PB could improve their growth and gut morphology better than antibiotics would, with a greater positive effect in the starter phase.