Tumor necrosis factor (tnf) is a pro-inflammatory factor and plays an important role in immune response to bacterial and viral infections and clearance of infected cells. In this study, zebrafish were intraperitoneally (i.p.) injected with immunostimulants, bacterial or viral pathogens, and the expression of tnfα and its receptor (tnf receptor superfamily (tnfrsf) member 1 a) was analyzed by real-time PCR in kidney and spleen. The results showed that tnfα and tnfrsf1a were constitutively expressed in all the tissues analyzed. Lipopolysaccharide (LPS) and polyinosinic acid-polycytidylic acid (poly(I:C)) modulated the expression of tnfα and tnfrsf1a. Intraperitoneal injection of Aeromonas hydrophila led to significant increases of tnfα expression in spleen (at 48 h and 72 h) and kidney (at 72 h) and that of tnfrsf1a in spleen (at 48 h). In contrast, in fish i.p. injected with Edwardasiella tarda, the tnfα expression increased significantly at all the time points whilst totally inhibited in spleen. Increase of tnfrsf1a expression was also seen in both tissues at 6 h and 72 h. The expression levels of tnfα and tnfrsf1a were significantly elevated at the early stage of infection with SVCV, with that of tnfrsf1a detected significantly higher in kidney of infected fish than that in the control group throughout the infection period (1-7 d). The results indicate that tnfα and tnfrsf1a play important roles in immune defence against bacterial and viral infections.