Two members of type II IFNs have been identified in fish, i.e. an IFN-gamma gene as in other vertebrates and a unique IFN-gamma related (IFN-gamma rel) gene being solely present in fish. However, the signalling pathways involved in the down-stream signalling of type II IFNs in fish remains poorly described. In this study, the type II IFNs mediated IRF1 was investigated in zebrafish, and the true homologous gene of mammalian IRF1 in fish was revealed despite the report of so-called IRF1a and IRF1b in zebrafish. As revealed in overexpression analysis, zebrafish IFN-gamma had a higher induction ability than IFN-gamma rel in relation with the expression of IRF1. IFN-gamma stimulated the expression level of STAT1a and also STAT1b, but they had opposite trends with the increase of time; enhancement of STAT1a waned after 12 h post injection of plasmids; whereas STAT1b expression increased continuously. Zebrafish IRF1 gene promoter contained several putative transcription factor binding sites, including GAS and NF-kappa B motifs. Luciferase assay revealed that the GAS site was essential in the IFN-gamma triggered IRF1 expression. In contrast, IRF11 contained neither GAS nor NF-kappa B elements, and did not respond to IFN-gamma induction. It is considered that STAT1a and STAT1b are structurally and functionally similar to STAT1 alpha and STAT1 beta in mammal respectively, and that IRF11, although used to be nominated as IRF1a, is not the orthologue of mammalian IRF1, but IRF1b in zebrafish should be the orthologue. (C) 2017 Elsevier Ltd. All rights reserved.