A novel IL-20-like (IL-20L) gene has been analyzed in rainbow trout. It has a six exon/five intron gene organisation and is translated into 181 amino acids (aa) with a signal peptide of 25aa and a mature peptide of 156aa. The trout IL-20L translation shares highest identities to other IL-20L molecules from fish and to IL-20 in mammals. Phylogenetic analysis showed that the fish IL-20L molecules form an independent clade but cluster with the group containing the IL-19, IL-20 and IL-24 molecules from higher vertebrates with a high bootstrap value (89%). The fish IL-20L genes are syntenically in the same location as mammalian IL-19/IL-20/IL-24. Gene organisation and multiple alignment also showed a close relationship of fish IL-20L genes to the mammalian IL-19/IL-20/IL-24 subfamily. These data suggest that the fish IL-20L molecules may have arisen from an ancestral gene that gave rise to IL-19, IL-20 and IL-24 in higher vertebrates. A high level of IL-20L expression in immune related tissues and in the brain, suggests an important role of the fish IL-20L molecule in both the fish immune system and nervous system. Although the exact cell types expressing IL-20L have yet to be defined, macrophages express IL-20L. Moreover, the IL-20L expression in the macrophage cell line RTS-11 was modulated by pro-inflammatory cytokines, signalling pathway activators, microbial mimics and the immuno-suppressor dexamethasone. These data suggest that trout IL-20L is an important player in the cytokine network. The increased expression of IL-20L was only detected at late stages (4-24h) of LPS stimulation in RTS-11 cells and in spleen 24-72h after infection with Yersinia ruckeri, and suggests that the increased expression of IL-20L by LPS and infection may be via the rapid increase of pro-inflammatory cytokines (e.g., IL-1beta) and other factors that is known to occur.