Interferon regulatory factor 1 (IRF1) was known to play key roles in antiviral defense in several species, and some other important biological processes. In this report, full length cDNA of IRF1 from Cynoglossus semilaevis (CsIRF1) was identified. It was of 1,455 bp, containing a 5' UTR of 104 bp, a 3' UTR of 541 bp with a poly (A) tail and an ORF of 810 bp encoding a putative protein of 269 amino acids. The putative CsIRF1protein contained one conserved IRF domain (1-113aa), and two low complexity regions (140-158aa and 230-242aa, respectively). Phylogenetic analysis showed that CsIRF1 was conserved in the teleost evolutionary branch, which was independent of mammalian, birds and amphibians. Additionally, CsIRF1 had the 96% homology with marine fishes, while 66% with freshwater fishes. The expression profiles of CsIRF1was analyzed by quantitative real-time PCR in healthy tissues and in immune tissues challenged with different pathogens [Vibrio anguillarum and Lymphocystis disease virus (LCDV)], respectively. CsIRF1 was widely expressed in healthy tissues of Cynoglossus semilaevis and with the highest expression in blood, as much as 19 times of that in liver. V. anguillarum and LCDV both induced the CsIRF1 gene expression distinctly in liver, with the peak value reached to 98-fold at 6 h and 25-fold at 24 h, respectively. The bacteria induced CsIRF1 suddenly up-expression in each detected tissues. However, at the initial stage of the challenge of virus LCDV, the CsIRF1 expression in blood and spleen were up regulated; on the contrary, its expression in liver and head kidney were down regulated, 0.3 and 0.4-fold 6 h post virus injection, respectively. These results suggested that CsIRF1 gene might involve in not only antiviral activity but also antibacterial procedure, indicating its vital role in Cynoglossus semilaevis innate defense system.