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Abstract Signal transducer and activator of transcription 1 (STAT1) is one of critical signal transduction proteins of interferon (IFN) pathway and the structure and function of this protein have been well identified in mammals, but the information about the STAT1 is still limited in teleost fishes. In the present study, the full-length cDNA sequence of STAT1 (Aj STAT1) in Japanese eel (Anguilla japonica) was identified and characterized. Multiple alignment of the amino acid sequence showed that the Aj STAT1 protein has the typical conserved domains including the amino-terminal, coiled-coil, DNA-binding, linker, Src homology 2 (SH2), transcriptional activation domains (TAD). Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed a broad expression for Aj STAT1 in a wide range of tissues, with the predominant expression in liver, followed by the spleen, intestine, gills, skin, kidney, and the very low expression in heart and muscle. The Aj STAT1 expressions in liver, spleen and kidney were significantly induced following injection with LPS, the viral mimic poly I:C, and Aeromonas hydrophila infection. In vitro , the Aj STAT1 transcripts of Japanese eel liver cells were significantly enhanced by the treatment of poly I:C or the stimulation of the high concentration of Aeromonas hydrophila (1 × 107 cfu/mL and 1 × 108 cfu/mL). Subcellular localization showed that in the natural state Aj STAT1was uniformly distributed in the cytoplasm, but Aj STAT1 was found to aggregated in the cytoplasm as well as partly in the nucleus after the stimulation of LPS and poly I:C. These results collectively suggested Aj STAT1 is an important transcription factor possibly involved in Japanese eel defense against viral and bacterial infection. Highlights • In vivo , Aj STAT1 was up-regulated by LPS, poly I:C, and bacteria. • In vitro , Aj STAT1 was enhanced by poly I:C and bacteria stimulation. • Aj STAT1 aggregated in the cytoplasm and the nucleus by stimulation of LPS and poly I:C. [ABSTRACT FROM AUTHOR]