In the present study, immune gene expression of rainbow trout, Oncorhynchus mykiss, against bacterial endotoxin (LPS) was studied in vivo. The expression of prointflammatory cytokine genes (IL-1$eta$ and TNF-$alpha$) and IFN-related genes (IRF-I, Mx-3) at gill was assessed by RT-PCR at different time point of day1 and day 3 post-injection. It was shown that the proinflammatory cytokine gene expression at gill was induced 1 day after LPS injection but the expression was not sustained until day 3. Meanwhile upregulated expression of IFN-related genes was found to be only at day 3 post injection, indicating indirect effect of LPS on these genes.