Using computer-based tools, an interleukin (IL) 10 homologue has been identified from the puffer fish ( Fugu rubripes) genome database. This is the first report on the existence of an IL-10 homologue in a non-mammalian vertebrate species. The Fugu IL-10 gene is located within a 2790-bp fragment including 549 bp of coding sequence which translates into an 183-amino-acid protein. It is predicted to contain five exons and four introns, sharing the same organization with the mammalian IL-10 genes. The size of the introns in the Fugu IL-10 gene is much smaller compared to mammalian IL-10 genes, whilst the size of the exons is similar. The deduced protein sequence shares 44-50% homology with the mammalian IL-10 sequences, 39-42% with the viral IL-10 sequences and 37-42% with other members of the IL-10 family, IL-20 and IL-22. Southern blot analysis indicates that a single copy of the IL-10 gene is present in the Fugu genome. A very low level of constitutive expression was detected in tissues of healthy fish including liver, kidney, gut and spinal cord, whilst no expression was detectable in spleen, gill, brain, gonad and eye. Analysis of the transcription regulation elements in the promoter region revealed that trans elements are located in the region between 1 bp and 721 bp, cis elements between 934 bp and 1114 bp and a tumour necrosis factor alpha responsive transcription element was located 92 bp upstream of the TATA box.