To date two closely related interleukin-1β genes (IL-1β1 and IL-β2) have been found in salmonids. The cloning of trout and salmon IL-1β3., and a salmon IL-1β4 pseudogene reveals that two types of IL-1β genes exist in teleost species. Type I teleost IL-1β genes, including salmonid IL-1β3, share a similar 6 coding exon structure as in tetrapods. Type II teleost IL-1β genes, e.g. salmonid IL-1β1-2, lack one or two coding exons at their 5'-end, and share higher identities within this subgroup than within the type I subgroup. Both types of IL-1β genes have been found in species of Salmoniformes, Perciformes and Beloniformes, suggesting the divergence occurred early in teleost evolution. Trout IL-1β3 is highly expressed in ovary suggesting a role in reproduction. A relatively high constitutive expression in gills, spleen and kidney and the up-regulation by PAMPs, proinflammatory cytokines and viral infection suggests IL-1β3 also has a role in inflammation and host defence.