seasonal variation of food items of bairdiella ronchus (cuvier, 1830) classified by size was analysed based on monthly samples between september/1993 and september/1996 at the guaratuba bay, southern brazil. the stomach contents of a hundred eighty-two fishes were analysed by the occunence frequency method and point count for a lot method. the individuais were divided in two groups, smaller and longer than 140mm, and a comparative study of digestive tract and branchial are was performed for these groups. for fishes smaller than 140mm the following sequence of items was obtained matching both methods by using the preponderance índices: decapods (subdivided in brachyura, caridea and penaeidea), polychaets, isopods, fishes, unidentified material, copepods, amphipods, molluscs and plant material. for fishes longer than or equal to 140mm the sequence was decapods, polychaets, fishes, copepods, amphipods, isopods, molluscs and plant material, in this order. these results suggest that bairdiella ronchus at the guaratuba bay is a carnivorous species using mainly decapod crustaceans as food throughout the year.