Four arginase isoforms exist in rainbow trout and Atlantic salmon. Synteny analysis revealed they are paralogues of arginase type I and II. In macrophages type I were modulated by PAMPs/cytokines but type II by Il-4/13. Vaccination/infection with Y. ruckeri also revealed isoform specific responses. Parasite infection (AGD in salmon, PKD in trout) down regulated both type I genes.