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1. This study was conducted to determine the effects of graded levels of phytase on the performance, egg quality and gut health of white laying hens. 2. Treatments consisted of a negative control (NC) diet containing 0.14% available phosphorus (avP), positive control (PC) diet containing 0.35% avP provided via dicalcium phosphate (DCP) and DCP replaced in the PC by with three graded levels of phytase derived from Komagataella phaffii at 500 (PC-500), 750 (PC-750) and 1000 (PC-1000) FTU/kg which provided 0.176%, 0.188% and 0.200% of avP, respectively. 3. Egg production, feed intake, feed conversion ratio and jejunal morphometry were negatively affected in NC-fed birds (p < 0.05). Considering the whole period, birds fed a diet supplemented with graded levels of phytase shared the same egg production and feed intake levels with PC birds (p < 0.05). Feed conversion ratio was significantly lowered by 4.9%, 1.6% and 7.6% in hens fed on diets PC-500, PC-750 and PC-1000, respectively compared to those fed the PC (p < 0.05). 4. Neither of the dietary treatments affected cracked eggs, dirty eggs, eggshell breaking strength and eggshell thickness. Dietary supplementation of phytase significantly increased villus surface area by 15%, 36% and 40% in PC-500, PC-750 and PC-1000 birds, respectively compared to PC (p < 0.05). 5. A significant increase in lactobacillus count was observed in line with increasing the level of phytase (p < 0.05). Dietary treatments had no effect on the caecal coliform or aerobic populations. Furthermore, phytase supplementation significantly increased the concentrations of total caecal short-chain fatty acid (SCFA; p < 0.01). 6. In conclusion, along with improving performance parameters, the inclusion of phytase in laying hen diets can ameliorate intestinal morphology and stimulate caecal microflora and increase SCFA concentrations. [ABSTRACT FROM AUTHOR]