Abstract Apparent digestibility coefficients (ADCs) of processed agro-food by-products were assessed in European seabass (Dicentrarchus labrax). Each experimental diet was obtained by replacing 300 g/kg of a commercial-based diet used as reference (REF) with a test ingredient: wheat germ (GERM), okara meal (OKA), poultry by-product meal (POULT), steam hydrolysed (FeHY) and enzyme-treated feather meal (FeENZ), beta-lactoglobulin (?-Lg) and peptide fractions >3,000 Da obtained from brewer's yeast (YeastP) and fish by-products (FishP). Dry matter ADC was highest in ?-Lg (95%) and lowest in OKA (40%). Protein ADCs were high in ?-Lg, FishP, GERM and POULT (>93%); intermediate in FeHY and FeENZ (85%?88%); and moderate in OKA and YeastP (70%?78%). The essential amino acids' ADC mean was above 91% in POULT, ?-Lg, GERM and FishP, 84%?89% in FeHY and FeENZ and 73%?76% in YeastP and OKA. Energy ADC was highest in POULT and ?-Lg (89%?95%) and lowest in YeastP and OKA (61%?64%). Lipid ADC was highest for POULT and GERM (100%). Phosphorus ADC was lowest in GERM (19%) and highest in ?-Lg (88%). FeHY, FeENZ, POULT, GERM, FishP and ?-Lg are highlighted as protein sources for European seabass.