The aim of this study was to investigate the effects of the addition of organic acid (OA), zeolite (ZE), or both to broiler diets with microbial phytase containing low and adequate levels of phosphorus (P) on performance, some serum indices, and ileum pH values. For the study, 480 broiler chicks 1-day old were assigned equally to 8 groups, with 6 replicates (10 chicks per replicate) carried out over 42 days. A basal diet containing 600 FTU of phytase enzyme was formulated separately for the starter and grower periods. Experimental groups were divided into 2 main groups: low P [(0.39% P; basal diet (B), B+OA, B+ZE, and B+OA+ZE)] and adequate P [(0.70% P; basal diet (B), B+OA, B+ZE, and B+OA+ZE)]. Th e average body weight of the low-P groups was signifi cantly lower than that of the adequate-P groups. However, birds fed ZE and OA+ZE had increased body weights. Within the fi rst 21 days of the study, the addition of ZE and OA+ZE to diets signifi cantly decreased feed intake, and feed conversion rates improved depending on the P level and the OA+ZE addition. Serum P levels increased depending on the decrease in the level of P in the diet at 21 days, and the ALP concentration at day 42 was found to be statistically lower in the group fed an OA+ZE diet. Ileum pH values in the OA group were found to be lower than those in other groups at 42 days. These results proved that the addition of ZE and OA+ZE to the diet with microbial phytase containing low and adequate levels of P had positive eff ects on performance. © TÜBİTAK.