Abstract: This study cloned the full-length of complement C7 cDNA by RACE (rapid-amplification of cDNA ends) from Russian sturgeon (Acipenser gueldenstaedti). It contained 3103 bp, including a 2502 bp open reading frame (ORF) encoding 833 dedueced amino acids, a 44 bp 5′-UTR and a 554 bp 3′-UTR. Multiple sequence alignment revealed that AgC7 proteins were well conserved with the typical structural motifs such as TSP1, LDLa, FIMAC, MACPF and had the sequence identity with Lepisosteus oculatus (56%), Ictalurus punctatus (46%), Danio rerio (49%), Salmo salar (49%) and Oreochromis niloticus (47%), respectively. AgC7 gene is constitutively expressed in all evaluated tissues with the highest expression in intestine and the weakest level in muscle. Chitosan oligosaccharide induced the expression of AgC7 gene to a certain degrees in all tissues with the highest increase in intestine (1.51 times). After infection with Aeromonas hydrophila, AgC7 gene exhibited distinct time-dependent response patterns in tested tissues with most obvious induction in gills and the maximum expression at 6h after infection (41.30 times). The expression of AgC7 gene decreased rapidly and returned to normal level after 12h. These results suggest that C7 of Russian sturgeon might play an important role in the immune defenses against bacterial infection.