Interleukin (IL)-11 is a multifunctional cytokine that stimulates hematopoi-etic progenitor cells and exerts a series of important immunomodulatory effects. It was believed to be restricted to mammals, but here we report the first nonmammalianIL-11 gene, in rainbow trout (Oncorhynchus mykiss). A trout IL-11 cDNA clone was isolated that contains a 5¢-untranslated region (UTR) of 400 bp, an open reading frame of 612 bp and a large 3¢-UTR of 1924 bp.