<title>Abstract</title>
In the intestine, the host must be able to control the gut microbiota and efficiently absorb transiently supplied metabolites, at the risk of enormous infection. In mammals, the inflammatory cytokine interleukin (IL)-17A/F is one of the key mediators in the intestinal immune system. However, many functions of IL-17 in vertebrate intestines remain unclarified.<bold><italic> </italic></bold>In this study, we established a gene-knockout (KO) model of IL-17 receptor A1 (RA1), an IL-17A/F receptor, in Japanese medaka (<italic>Oryzias latipes</italic>) using genome editing technique and the phenotypes were compared to wild type (WT) based on transcriptome analyses.<bold><italic> </italic></bold>Upon hatching, homozygous IL-17RA1-KO medaka mutants showed no significant morphological abnormality. However, after 4 months, significant weight decreases and reduced survival rates were observed in IL-17RA1-KO medaka. Comparing gene-expression patterns in WT and IL-17RA1-KO medaka revealed that various metabolism- and immune-related genes were significantly down-regulated in IL-17RA1-KO medaka intestine, particularly genes related to mevalonate metabolism (<italic>mvda</italic>, <italic>acat2</italic>, <italic>hmgcs1</italic>, and <italic>hmgcra</italic>) and genes related to IL-17 signaling (such as <italic>il17c</italic>, <italic>il17a/f1,</italic> and <italic>rorc</italic>) were found to be decreased. These findings show that IL-17RA regulated immune- and various metabolism-related genes in the intestine for maintaining the health of Japanese medaka.