In this study, full-length tilapia transferrin (OnTF) isolated from liver cDNA of Nile tilapia (Oreochromis niloticus) was found to have an open reading frame of 2,091-bp encoding 696 amino acid residues. Two additional amino acids: Gly(369) and Gly(370) were observed compared with the reported Nile tilapia transferrin protein sequence. Pre-mature protein has a predicted molecular weight of 78.2 kDa, while mature protein is 73.28 kDa in size. Comparative sequence analysis with transferrin from other species revealed two major putative iron-binding domains designated as the N-lobe and the C-lobe in accordance with the transferrin protein characteristics. The predicted tertiary structure of tilapia transferrin confirmed the presence of iron and anion-binding sites on both lobes that are conserved among transferrins from other species. Quantitative real-time PCR analysis showed significantly higher expression of tilapia transferrin gene in liver than in other tissues (p < 0.05). Transferrin expression in tilapia experimentally infected with 10(6) and 10(8) colony-forming units mL(-1) of Streptococcus agalactiae was significantly upregulated at 24 and 12 h post-infection (hpi), respectively, and decreased afterward. Iron-deficiency in serum of bacterially infected fish was detected at 48 and 24 hpi, respectively. The expression pattern of the transferrin gene and the iron levels of infected tilapia in this study were consistent with the function of transferrin in innate immunity.