| Summary: | This study investigated the growth performance, serum immunity, and cecal bacterial microbiota of broilers fed a diet in which soybean meal (SBM) was partially replaced with fermented soybean meal (FSBM) for 36 days. A total of 180 one-day-old male Cobb 500 broilers were randomly divided into three dietary groups (six replicates per group): corn-SBM diet (CC); 25% SBM replaced by FSBM (SC); 50% SBM replaced by FSBM (TC). The average daily gain (ADG) and feed conversion rates (FCR) were higher in SC than CC and TC groups (<i>p</i> < 0.05) during the growth (d 22–36) and whole (d 1–36) phases. No significant difference was observed in ADG and average daily feed intake (ADFI) between CC and TC groups during any phases. Dietary treatments increased serum IgA, IgG, and IgM, Chao 1, observed species, and the abundance of the phylum <i>Fimicutes</i> but decreased the proportion of <i>Proteobacteria</i> (<i>p</i> < 0.05). Dietary treatments increased the abundance of the genera <i>Lachnospiraceae</i>, <i>Lachnoclostridium</i>, <i>Gastranaerophilales</i>, and <i>Lactobacillus</i> but decreased the abundance of <i>Escherichia-Shigella</i> and <i>Clostridiales</i> (<i>p</i> < 0.05). Spearman’s correlations showed that the abundance of <i>Gastranaerophilales</i> was positively correlated with ADG and serum immunity, and the abundance of <i>Lactobacillus</i> was strongly positively with IgM. Thus, replacing 25% of SBM with FSBM improves the growth performance and serum immunity of broilers, possibly due to altered cecal microbial composition.
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