Peroxiredoxins (Prxs) constitute a superfamily of antioxidative proteins that play important roles in protecting organisms against damage from reactive oxygen species (ROS).We report the isolation of peroxiredoxin V gene from Argopecten irradians (designated Ai-PrxV).The full length Ai-PrxV cDNA consists of 1689 bp with a 567 bp open reading frame (ORF) that encodes 188 amino acids.Three putative polyadenylation consensus signals (AATAAA) were found in the 953 bp long 3-UTR.The genomic length of the Ai-PrxV gene is 12575 bp, and it contains six exons and five introns.The gene structure is closely related to that of chordates but differs from that of arthropods.The first intron of Ai-PrxV is of 8565bp, containing an endonuclease/reverse transcriptase sequence truncated at the 5 end and many repeat sequences.We propose there was involvement of a retrotransposon in the evolution of the PrxV gene.Quantitative reverse transcriptase real-time PCR (qRT-PCR) analysis showed that the highest expression of the Ai-PrxV transcripts occurred in gill tissue.When challenged with the bacteria Vibrio anguillarum, the level of Ai-PrxV transcripts in hemocytes of bay scallops was greatest at 15 h after injection.These results indicate that Ai-PrxV is a constitutive and inducible protein that plays an important role in the immune response against bacterial infection.