Most loricariids are algivorous and detritivorous and play an important role in both the grazer and detritus food chains of neotropical waters. Relationships between morphological variation and diet were analysed in six syntopic species (Rhinelepis aspera, Hypostomus regani, H. ternetzi, H. margaritifer, H. microstomus and Megalancistrus aculeatus) fished commercially in a 10 km2 area of the Upper Paraná River upstream from the Itaipu Reservoir. Species feeding on fine grained detritus use suction to obtain food, and possess a well-developed respiratory membrane, long gill rakers, rudimentary labial and pharyngeal teeth, a thin stomach wall, and a long intestine. Species feeding on coarser material (e.g. periphyton), ingest food by scraping the substratum. Their morphological characteristics are large, strong, spatulate teeth, short gill rakers, a well-developed stomach, and a shorter intestine. The first species group is composed by R. aspera while the latter by M. aculeatus and H. microstomus. The others species had an intermediary position.