In order to investigate the structure and expression of vtg encoding vitellogenin (VTG) in Osteoglossum bicirrhosum, full-length cDNA sequence of vtg was cloned by the rapid-amplification of cDNA ends (RACE). Sequence analysis found that the full length of vtg was 5 325 bp, including 5 121 bp of the open reading frame (ORF), 46 bp of the 5′ untranslated region (UTR) and 159 bp of 3′ UTR. The ORF contained a total of 1 706 encoded amino acids with a predicted molecular mass of 186.79 ku. The results of domain analysis showed that there were 4 conserved domains in ORF, lipoprotein amino terminal region (vitellogenin-N), von Willebrand factor type D domain (vWD), domain of unknown function (DUF) 1943 and DUF 1944 (Fig. 2). The results of homologous analysis showed that the similarity comparing with Scleropages formosus was the highest of 84.78%, and higher than 50% comparing with Cypriniformes, Perciformes, Clupeiformes, Heterosomata. Phylogenetic analysis showed that vtg of O. bicirrhosum came to be a cluster with Osteoglossiformes, and was closely related with Anguilliformes, which was consistent with the evolutionary position of O. bicirrhosum (Fig. 5). The expression pattern of vtg was evaluated using real-time fluorescent quantitative PCR, and the results showed that the expression of vtg was the highest in the liver of female, significantly higher than that in males (P < 0.05). There was no significant difference in its expression between ovary and testis (P > 0.05). The expression quantity of vtg in gills, spleen, kidney, muscle, heart, head-kidney or brain of males and females was all very low (P > 0.05) (Fig. 6). The relative expression of vtg in the ovary at stage Ⅱ, Ⅲ and Ⅳ was increased successively, and its expression at the stage Ⅳ was significantly higher than that at stage Ⅱ and Ⅲ (P < 0.05), and was significantly higher than that of stage Ⅳ testis. In the liver of female vtg expression was increased at first and then decreased, and no significant difference among the 3 stages (P > 0.05) was found, but vtg expression at each stage was significantly higher than that of males (P < 0.05) (Fig. 7). We successfully constructed the prokaryotic expression vector, and transferred it to Transetta (DE3) expression competent cells to induce expression the fusion protein (Fig. 8, 9). Western-blotting showed that the fusion protein could be specifically recognized by anti-His tag (Fig. 10). In conclusion, the expression level of vtg is related to sex, and it is also related to gonadal development stage. In addition, this study has laid a foundation to further investigating the physiological function of VTG.