Ontogenetic diet shifts of the dusky grouper, Epinephelus marginatus (Lowe, 1834), in the Balearic Islands (western Mediterranea) were studied using the stomach contents of 203 specimens ranging between 134 to 1056 mm total length. A total of 64.5% of the examined stomachs contained food. Diet composition, characteristics of ingested preys (average number and average weight), feeding strategy and niche overlap in relation to fish size were evaluated. There were statistical differences (Redundancy Analysis) for the proportions of the main food categories related to size. A high proportion of crustaceans was characteristic of the diet of smaller fishes, while the proportion of cephalopods increased for the larger sizes. Nevertheless, this pattern only represented a low percentage (5.5%) of the variation in diet composition. For growth, the dusky grouper increased the ingestion of larger preys rather than catch a greater number of preys. In proportion to a larger mouth size there was an increased ability to catch voluminous cephalopods and, as a result, the grouper developed from a generalist to a more specialised type of feeding. Shifts in diet composition could be related to behavioural changes. When size is a good predictor of diet composition, the function derived from Redundancy Analysis could be used to estimate the representative diet of a stock in relation to its size structure.