LGP2 (laboratoryofgeneticsandphysiology2), ahomologueof RIG-I (Retinoicacidinduciblegene-I)and MDA5 (Melanomadifferentiationassociatedgene5)withouttheCARD (caspaseactivationandrecruitment domain)requiredforsignaling, playsapivotalroleinmodulatingsignalingby RIG-I andMDA5 for interferon (IFN)synthesis. Inthisstudy, anovel LGP2 genefromgrasscarp Ctenopharyngodonidella (designatedas CiLGP2) wasisolatedandcharacterized. Thefull-lengthcDNAof CiLGP2 wasof2920bpwith fiveinstability motifs (ATTTA). Theopenreadingframewasof2043bpandencodedapolypeptideof680aminoacids, including fivemainoverlappingstructuraldomains:twoDEXDc (DEAD/DEAHboxhelicasedomain), one ResIII (conservedrestrictiondomainofbacterialtypeIIIrestrictionenzyme), oneHELICc (helicase superfamilyc-terminaldomain)andoneRD (regulatorydomain). Therewasonemore a-helixintheRD, comparedwiththatinhuman. The CiLGP2 mRNAwasubiquitousexpressioninthetestedtissues, washigh levelinspleen, skin, heartandintestinetissues, andwasup-regulatedbygrasscarpreovirus (GCRV) injectionbysemi-quantitativeRT-PCR (sqRT-PCR)assay. The CiLGP2 expressioninspleenwassignificantly up-regulatedat12h (14.5 folds, P<0.05), reachedthecrestat24h (19.0 folds, P<0.05), andthendropped a littleat48h (10. 4folds)post-injectionofGCRVandkeptthislevelinthefollowingtestperiod (P<0.05). In liver, thetemporalexpressionof CiLGP2 mRNAwassignificantlyincreasedat24h (3.8 times, P<0.05), reachedpeakat 48h (10.7 times, P<0.05), andthendecreasedalittlebitat72h (5.8times, P<0.05) and keptthishighlevelbytheendofthetest (P<0.05). Theseresultscollectivelysuggestedthat CiLGP2 was a novelmemberofRLRgenefamily, engagingintheearlystageofantiviralinnateimmunedefenseingrass carp, andlaidthefoundationforthefurthermechanismresearchof LGP2 in fishes.