The shortcomings and merits of the four principal methods used to determine the trophic role of fishes (frequency of occurrence, numerical, volumetric, and gravimetric methods) are examined. Limitations of each method are illustrated using original material on feeding habits of fishes off Peru. The authors assert that, to determine the trophic role of fishes in ecosystems of marginal seas and the World Ocean, the gravimetric method is the most appropriate.