The present study investigated the effects of dietary trans-cinnamic acid (CA) on pre- and post-challenge haematological, serum biochemical, non-specific immune and head kidney gene expression responses of rainbow trout, Oncorhynchus mykiss juveniles. In this regard, fish with an average weight of 17.01 +/- 0.05 g were divided into five groups, and fed daily with an additive free basal diet (control); 250, 500, 750 or 1500 mg kg(-1) CA for a 60-day period. Fish were sampled every 20 days during the experiment. On days 20, 40 and 60 (the pre-challenge period), the dietary CA especially at 250 and/or 500 mg kg(-1) significantly increased blood granulocyte percentage, and serum total protein, globulin, lysozyme and total immunoglobulin values. Furthermore, dietary CA increased activities of phagocytic activity, respiratory burst and potential killing, and increased the expression levels of immune related genes [serum amyloid A (SAA), interleukin 8 (IL-8), interleukin 1, beta (IL-1 beta), transforming growth factor beta (TGF-beta), tumor necrosis factor (TNF-alpha), and immunoglobulin T (IgT)] in the head kidney of fish fed with 250 and/or 500 mg kg(-1) CA. Following 60 days of feeding, fish were challenged with Yersinia ruckeri and mortality was recorded for 20 days. Highest percentage survival (%) rate was found in the 250 and/or 500 mg kg(-1) CA-supplemented feeding groups. During the post-challenge period, red blood cell (RBC) count, hematocrit (%), respiratory burst activity, and total antiprotease activity increased in fish fed with feed containing 500 mg kg(-1) content. Moreover, markedly up-regulated the expression of related genes (SAA, IL-8, IL-1 beta, TGF-beta, TNF-alpha, IFN-gamma and IgM) in fish fed 250, 500 and/or 750 mg kg(-1) CA. Therefore, feeding 0. mykiss for 60 days with dietary CA at 250-500 mg kg(-1) CA incorporation levels can be suggested as optimal to enhance the immunity and disease resistance against Y. ruckeri.