We cloned the DNA and cDNA sequence of the Pentraxin 3 gene (PTX3) from Carassius auratus,and examined its expression patterns in various tissues and after Aeromonas hydrophila infection.Bioinformatics analysis showed that the C. auratus PTX3 gene consists of three exons and two introns,encoding 453 amino acids with the signal peptide (1-22 aa) and a pentraxin domain (241-453 aa).Sequences analysis revealed that the C. auratus PTX3 amino acids sequence is closest to that of zebrafish (79%).Phylogenetic analysis showed that C. auratus fall in the fish clade.Real-time quantitative PCR results showed that the C. auratus PTX3 is constitutively expressed in the heart,liver,spleen,head kidney,kidney,gills,brain,muscle,intestine,blood,and gonads.The mRNA levels of PTX3 in the liver,spleen,head-kidney,intestine and blood increased significantly after A. hydrophila infection.At 3 h post-infection,the PTX3 mRNA levels in spleen were significantly increased (P<0.05) and at 6 h post-infection,the PTX3 mRNA levels in the blood reached a peak.These results indicated that PTX3 is an acute phase protein which can protect fish against infections.