To explore the possible function of the complement factor B and C2 (Bf/C2) gene in blunt snout bream (Megalobrama amblycephala), the cDNA sequences were cloned with RT-PCR and the expression of the Bf/C2mRNA in healthy fish tissues and after Aeromonas hydrophila infection during early development was investigated with quantitative realtime PCR.The results showed that the sequence length of Bf/C2A cDNA was 2 520 bp containing a 42 bp 5′ UTR,a 2 298 bp ORF and a 180 bp 3′ UTR,encoding 765 amino acid.The Bf/C2B cDNA had an 2 131 bp ORF,encoding 710 amino acid.The mRNA of both Bf/C2A and Bf/C2B were highly expressed at 1 dpf,followed by the intestinal formation stage during early development.The highest expression of both Bf/C2A and Bf/C2B in healthy adults appeared in the liver,followed by the kidney,head kidney and spleen.Theexpression of Bf/C2A and Bf/C2B exhibited a significant change after infection of A. hydrophila in the inmmunityrelated tissues including the liver and kidney,indicating that Bf/C2 plays important role in the immune response against bacterial infection.