A homologue of interleukin 18 has been identified from rainbow trout, Oncorhynchus mykiss. The trout IL-18 gene spans 3.7 kb and consists of six exons and five introns, sharing the same gene organization with its human counterpart. The putative translated protein is 199 amino acids in length with no predicted signal peptide. Analysis of the multiple sequence alignment reveals a conserved ICE cut site, resulting in a mature peptide of 162 amino acids. The trout IL-18 shares 41-45% similarity with known IL-18 molecules and contains an IL-1 family signature motif. It is constitutively expressed in a wide range of tissues including brain, gill, gut, heart, kidney, liver, muscle, skin and spleen. Transcription is not modulated by lipopolysaccharide, poly(I:C) or trout recombinant IL-1beta in primary head kidney leucocyte cultures and RTS-11 cells, a macrophage cell line. However, expression is downregulated by lipopolysaccharide and rIL-1beta in RTG-2 cells, a fibroblast-like cell line. An alternatively spliced form of IL-18 mRNA has also been found and translates into a 182 amino acid protein with a 17 amino acid deletion in the precursor region of the authentic form. This alternatively spliced form is also widely expressed although much lower than the authentic form. Interestingly, its expression is upregulated by lipopolysaccharide and poly(I:C), but is not affected by rIL-1beta in RTG-2 cells. The present study suggests that alternative splicing may play an important role in regulating IL-18 activities in rainbow trout.