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To investigate the effects of Kluyveromyces marxianus on immune responses, intestinal structure and microbiota in broilers, 840 1-d-old broiler chicks were randomly divided into seven groups (eight replicates) and were fed basal diets without or with 0.25, 0.50, 1.0, 1.5, 2.0, and 2.5 g/kg of K. marxianus (2.0×1010 CFU/g). Serum and intestine samples were collected at 21 d of age. The results showed that increasing K. marxianus addition linearly reduced feed conversion ratio but linearly elevated relative thymus weight, as well as quadratically increased serum lysozyme and IgG levels, with the medium dose (1.0 g/kg) being the most effective. The ratio of villus height to crypt depth of jejunum and ileum, ileal villus height and sucrase activity, as well as the mRNA expression of ileal mucin-2, claudin-1 and sodium glucose cotransporter 1 linearly responded to the increasing K. marxianus addition. Supplemental K. marxianus at low (0.5 g/kg), medium (1.5 g/kg) and high (2.5 g/kg) dose all decreased the abundance of phylum Cyanobacteria, increased the abundance of phylum Firmicutes and genus Lactobacillus in ileum. The high dose of K. marxianus addition also reduced the abundance of order Rickettsiales and Pseudomonadales along with species Acinetobacter junii. Ileal bacterial communities between K. marxianus-treated and untreated groups formed distinctly different clusters. In summary, K. marxianus supplementation benefits feed efficiency and immune function, as well as intestinal structure in broilers, which might be attributed to the improved ileal microbial structure. Supplemental K. marxianus at high dose (2.5 g/kg) was more effective for feed efficiency and intestinal health of broilers, while the innate immunity was optimized at a medium dose (1.0 g/kg). [ABSTRACT FROM AUTHOR]