This study was designed to explore the effect of protein source on muscle susceptibility to lipid peroxidation in two representative species of fish farmed for human consumption: the freshwater rainbow trout and the seawater European sea bass. Four isoproteic diets (digestible protein in the range 366 to 392 for rainbow trout and 391 to 415 g/kg for European sea bass) were formulated to contain one of the following as the main protein source: fish meal, warm water alcohol-extracted or toasted soya protein concentrates or maize gluten meal. Highest daily growth indices were always achieved using the diets based on fish meal as the main source of protein (P < 0·05). Fish of both species given diets containing maize gluten and the toasted soya protein concentrate showed slowest growth. The depressant growth effect of the vegetable protein concentrates was greater in sea bass than in rainbow trout. Dietary treatment was not correlated with any significant effect on whole-body composition or intramuscular fat content except for ash concentration in European sea bass. Under conditions of forced peroxidation in vitro for 240 min, muscle specimens of trout and sea bass given diets containing fish protein as the main source of protein showed the highest peroxidation levels (P < 0·05); while the lowest peroxidation values were found in fish given maize gluten-containing diets (P < 0·05). In the present case, the partial substitution of fish meal with vegetable proteins in diets led to a lower susceptibility of fish flesh to peroxidation. This finding may have applications in the production of fish of improved quality and longer shelf life.