The experiment was conducted to study how adding rice hulls and an acidifier to the diet affected perform-ance, histomorphology, and microbial colonization in the gastrointestinal system of broiler chickens. A completely randomized design was used to test eight treatments and five replications of 1000 day-old-male Arbor Acres broiler chickens (45±1 g). Dietary treatments included control (no additives 1-42 days), acidi-fier (control+0.1% acidifier from 1-42 days), RH10 (3% rice hull from 1-10 days and control diet from 11-42 days), RH10 + acidifier (3% rice hulls from 1-10 days and control diet+0.1% acidifier from 11-42 days), RH24 (3% rice hulls from 1-24 days and control diet from 25-42 days), RH24 + acidifier (3% rice hulls from 1-24 days and control diet+0.1% acidifier from 25-42 days), RH42 (3% rice hulls from 1-42 days) and RH42 + acidifier (3% of the rice hulls+0.1% acidifier from 1-42 days). Feed conversion ratio significantly decreased (P<0.05) in the RH24 + acidifier group compared with RH10 and control groups after 24 and 42 days. The RH42 + acidifierbirds had lower mortality than RH24+acidifier birds (P<0.05). Also, at 42 days of age, feeding RH42 + acidifier diet increased villus height/crypt depth (P<0.01). Acidifiers increased the Lactobacillus population in the small intestine of broiler chickens. The lowest gizzard pH was observed in birds fed 3% rice hulls from 0 to 42 days of age (P<0.05). In general, the addition of 30 kg/ton of rice hulls to the broiler feed, from 0 to 42 days of age, caused the highest possible weight gain and the lowest mortal-ity. Manuscript profile