This work is the first description of Mycobacterium peregrinum as an etiological agent for mycobacteriosis in farmed fishes. We report the mycobacterial infection in farmed European tench (Tinca tinca L.) which was confirmed by culture, molecular identification methods (PCRs aimed at 16S rRNA, rpobeta and hsp65 sequencing), and histopathology. Since M. peregrinum infection has been described in humans, their clinical significance in fishes should be considered of healthcare interest. With this case report, we also show that a multidisciplinary approach was needed to overcome difficulties associated to diagnosis of piscine mycobacteriosis.