The investigation and application of a wide range of dietary supplements, such as probiotics, prebiotic and other additives, are increasingly popular in aquaculture research and practice. To date few studies have attempted to quantify the value of commercially available additives in improving growth performance of juvenile turbot (. Scophthalmus maximus) and in compensating potential growth reduction resulting from high levels of plant protein (PP) in carnivorous fish diets.Two experiments were conducted to investigate the effect of different active ingredients in diet additives on turbot. I) Five diets supplemented with (1) yeast b-glucan and mannan oligosaccharides (GM), (2) alginic acid from brown algal extracts (AC), (3) yeast nucleotides and RNA (NR), (4) potassium diformate (PDF) and (5) bacteria strains Bacillus subtilis and B. licheniformis (BS), containing fish meal (FM) as the only protein source, were fed to turbots (initial weight 48.8. g. ±. 5.2. g) over 112. days. II) Four diets supplemented with (1) GM, (2) AC, (3) NR and (4) BS, containing soy protein concentrate (SPC) and wheat gluten (WG) as a partial replacement of FM, were fed to turbots (initial weight 95.8. g. ±. 17.7. g) over 84. days. A non-supplemented FM diet (exp. I) and an FM- and PP-based diet (exp. II), respectively, were used as control diets.Diet additives did not promote additional weight gain, specific growth rate (SGR), daily feed intake (DFI) and feed conversion ratio (FCR) in turbot fed FM- or PP-based diets (p. >. 0.05) when compared to isocaloric control diets in both experiments. Growth of turbots fed the high FM content control diet (II) was significantly higher than all other treatments (p. <. 0.01). Body proximate composition, condition factor (K) and liver index (HSI) remained unaffected by additive supplementation in fish fed either FM or PP diets (p. >. 0.05).Results indicate that reported benefits for specific diet additives cannot be assumed to function or applied across species boundaries and age classes. In addition, dietary additive application may not be economically valid for larger animals and/or animals not exposed to specific culture-related stressors. The benefits of popular additives to high value species such as S. maximus remains to be tested under specific immune or physical stress situations and at crucial larval and early juvenile stages.