IPS-1 (interferon-b promoter stimulator 1), also known as MAVS/VISA/Cardif, plays a central role in antiviral immunity. In this manuscript, we cloned and characterized IPS-1 from grass carp Ctenopharyngodon idella (designated as CiIPS-1). The CiIPS-1 cDNA is 2412 bp long and consists of a 50 untranslated region (UTR) of 124 bp, a 30 UTR of 497 bp with three cytokine RNA instability motifs (ATTTA) and a polyadenylation signal (AATAAA), and an open reading frame (ORF) of 1791 bp encoding a polypeptide of 596 amino acids with a calculated molecular mass of 64.1 kDa and a theoretical isoelectric point of 4.79. Structural analysis showed that the CiIPS-1 protein contained an N-terminal CARD (caspase activation and recruitment domain), a central proline-rich domain, a putative TRAF2-binding motif and a C-terminal transmembrane domain. Similarity analysis of the deduced amino acid sequence of the CiIPS-1 by MatGAT software revealed that the CiIPS-1 shared 27.8e76.4% identity and 47.4e85.2% similarity with other known piscine IPS-1 sequences. The CiIPS-1 mRNAwas constitutively expressed in the examined tissues, higher in spleen, and was induced by grass carp reovirus (GCRV) injection by semi-quantitative RT-PCR assay. Quantitative real-time RT-PCR analysis revealed that the CiIPS-1 mRNA expression was rapidly and significantly up-regulated in vivo and in vitro after GCRV infection, and the CiIPS-1 transcripts were also significantly enhanced in vitro post the synthetic double stranded RNA polyinosinicepolycytidylic potassium salt (poly (I:C)) stimulation. These results indicated that CiIPS-1was an inducible acute-phase protein and involved in the immune reaction to GCRV in grass carp.