In this study, the effect of the methanolic extract of tetra (Cotinus coggygria) against Vibrio anguillarum in cultured koi carp (Cyprinus carpio carpio) was investigated. Three different concentrations of tetra extract (0, 0.5, 1, 1.5 g kg(-1) of feed) were individually mixed with the basal diet and fed to koi (average body weight of 4,14 +/- 0,08 g) for a 4-week period to assess the immunomodulatory characteristics of tetra. At the end of the study, koi was challenged with a strain of Vibrio anguillarum (10(8) CFU ml(-1)). Growth performance was not affected by dietary tetra extract intake (P>0.05). Nitroblue tetrazolium (NBT) activity was found to be the highest in the 1.5 g kg(-1) containing group. Lysozyme and myeloperoxidase activity of the experimental groups was significantly improved in the tetra groups compared to the control group (P<0.05). Red blood cell and mean corpuscular haemoglobin concentration (MCHC) were higher in 1 g kg(-1) tetra containing group (P<0.05). This study shows that the koi treated with 0, 0.5, 1 and 1.5 g kg(-1) tetra extract prior the challenge with live V. anguillarum had 37.50%, 31.94 %, 18.06 % and 12.50 % mortality. According to study results, tetra is an effective non-specific immunostimulant for koi.