The suppressor of cytokine signaling 3 (SOCS-3) is a member of a newly discovered protein family, which have been shown to regulate the responses of many immune cytokines, such as interferon (IFN), interleukin-2 (IL-2) and IL-6, etc., by inhibiting Janus kinase (JAK)-signal transducers and activators of transcription (STAT) signaling in a negative auto-regulatory manner. Although SOCS-3 was well characterized in several mammal species, there was still no report in fish. In present study, we initially identified and characterized the SOCS-3 genes from three fishes, the Tetraodon nigroviridis, the Danio rerio and the Fugu rubripes. The results showed that Tetraodon SOCS-3 gene located within a 2666 bp genomic fragment of chromosome 3, transcribed into a 1445 bp mRNA including 273 bp 5' UTR (untranslated region), 606 bp ORF (open reading frame) and 566 bp 3' UTR. Tetraodon SOCS-3 with 201aa (amino acid) has a calculated molecular mass of 22.76 kDa and a theoretical pI of 8.99. Danio SOCS-3 gene located within a 3617 bp genomic fragment of chromosome 3, transcribed into a 1927 bp mRNA including 178 bp 5' UTR, 624 bp ORF and 1125 bp 3' UTR. Danio SOCS-3 with 207aa has a calculated molecular mass of 23.68 kDa and a theoretical pI of 9.19. Fugu SOCS-3 gene located within a 2842 bp genomic fragment of Scaffold_1118, transcribed into a 1528 bp mRNA including 209 bp 5' UTR, 606 bp ORF and 713 bp 3' UTR. Fugu SOCS-3 with 201aa has a calculated molecular mass of 22.76 kDa and a theoretical pI of 8.18. The fish SOCS-3-encoding genes with the same organization as the mammalians consist of two exons and a single intron that lies in the 5' UTR of the transcript. The deduced amino acid sequences of the fish SOCS-3s showed: 60.7-61.7% sequence identity to mammalian SOCS-3s; 62.3-63.2% sequence identity to bird SOCS-3s; and 55.3-57.8% sequence identity to amphibian SOCS-3s. Phylogenetic analysis separates the fish SOCS-3s into an exclusive group. Expression study of Tetraodon SOCS-3 mRNA in ten selected tissues showed that it was constitutively expressed and induced by lipopolysaccharide (LPS) strikingly. These results indicated that SOCS-3s in fish may be involved in inflammatory responses. This is the first report of cloning and characterization of SOCS-3 cDNAs and genes in fish.